*Review* **Recent Advances of Interventional Endoscopic Retrograde Cholangiopancreatography and Endoscopic Ultrasound for Patients with Surgically Altered Anatomy**

**Yuki Tanisaka \*, Masafumi Mizuide, Akashi Fujita, Tomoya Ogawa, Masahiro Suzuki, Hiromune Katsuda, Youichi Saito, Kazuya Miyaguchi, Tomoaki Tashima, Yumi Mashimo and Shomei Ryozawa**

> Department of Gastroenterology, Saitama Medical University International Medical Center, 1397-1, Yamane, Hidaka, Saitama 350-1298, Japan; mizuide1971@yahoo.co.jp (M.M.); a.fujita0628@gmail.com (A.F.); t.ogawa0210@icloud.com (T.O.); msuzgast@tmd.ac.jp (M.S.); hk0112@saitama-med.ac.jp (H.K.); stm\_ys41@yahoo.co.jp (Y.S.); kaz.hr77@gmail.com (K.M.); tomo3029@saitama-med.ac.jp (T.T.); ymashimo@saitama-med.ac.jp (Y.M.); ryozawa@saitama-med.ac.jp (S.R.)

**\*** Correspondence: tanisaka1205@gmail.com; Tel.: +81-42-984-4111

**Abstract:** Endoscopic retrograde cholangiopancreatography (ERCP) is considered to be the gold standard for diagnosis and interventions in biliopancreatic diseases. However, ERCP in patients with surgically altered anatomy (SAA) appears to be more difficult compared to cases with normal anatomy. Since the production of a balloon enteroscope (BE) for small intestine disorders, BE had also been used for biliopancreatic diseases in patients with SAA. Since the development of BE-assisted ERCP, the outcomes of procedures, such as stone extraction or drainage, have been reported as favorable. Recently, an interventional endoscopic ultrasound (EUS), such as EUS-guided biliary drainage (EUS-BD), has been developed and is available mainly for patients with difficult cases of ERCP. It is a good option for patients with SAA. The effectiveness of interventional EUS for patients with SAA has been reported. Both BE-assisted ERCP and interventional EUS have advantages and disadvantages. The choice of procedure should be individualized to the patient's condition or the expertise of the endoscopists. The aim of this review article is to discuss recent advances in interventional ERCP and EUS for patients with SAA.

**Keywords:** endoscopic retrograde cholangiopancreatography; altered anatomy; ERCP; balloon enteroscope; single balloon enteroscopy; double balloon enteroscopy; endoscopic ultrasound; EUS; interventional EUS; EUS-BD

### **1. Introduction**

There is a large variety of biliary tract diseases, such as bile duct stones and benign/malignant biliary strictures. They lead to hepatobiliary dysfunction, cholangitis, and eventually liver failure requiring appropriate therapy. Since its introduction in 1968, endoscopic retrograde cholangiopancreatography (ERCP) is thought to be the gold standard for diagnosis and interventions in biliopancreatic diseases. It has been reported that ERCP-related procedures have achieved success in approximately 95% of cases [1,2]. However, it is technically challenging to perform ERCP in patients with surgically altered anatomy (SAA), such as Roux-en-Y gastrectomy, hepaticojejunostomy with Roux-en-Y, pancreaticoduodenectomy, or Billroth II gastrectomy. First of all, the afferent limb, increased intestinal curvature, or postoperative adhesions hinder accessibility of the target site, such as the papilla or the hepatico/pancreatojejunal anastomosis. Next, selective biliary cannulation and subsequent procedures, such as stone extraction or drainage, are more difficult in patients with SAA than cases with normal anatomy. Outcomes using a conventional duodenoscope have not been satisfactory [3,4]. Hence, alternative treatments, such as percutaneous transhepatic biliary drainage (PTBD), have been widely applied to patients with SAA [5,6]. One study from a tertiary referral endoscopy center reported

**Citation:** Tanisaka, Y.; Mizuide, M.; Fujita, A.; Ogawa, T.; Suzuki, M.; Katsuda, H.; Saito, Y.; Miyaguchi, K.; Tashima, T.; Mashimo, Y.; et al. Recent Advances of Interventional Endoscopic Retrograde Cholangiopancreatography and Endoscopic Ultrasound for Patients with Surgically Altered Anatomy. *J. Clin. Med.* **2021**, *10*, 1624. https:// doi.org/10.3390/jcm10081624

Academic Editor: Saburo Matsubara

Received: 23 February 2021 Accepted: 9 April 2021 Published: 12 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that the afferent loop intubation and cannulation success rates using side-viewing duodenoscope in patients with Billroth II gastrectomy were 86.7% (618/713 patients) and 93.8% (580/613 patients). The main reason for intubation failure was a long and angulated afferent loop [7]. Another systematic review and meta-analysis reported that the afferent loop intubation and cannulation success rates using a forward-viewing endoscope in patients with Billroth II gastrectomy were 91.1% and 92.3%. The subgroup analysis of the forward-viewing endoscope showed that the success rates of afferent loop intubation using the forward-viewing endoscope with cap-fitting (92.5%) was higher than the forwardviewing endoscope without cap-fitting (88.6%). The success rates of cannulation using the forward-viewing endoscope with cap-fitting (93.7%) was higher than the forward-viewing endoscope without cap-fitting (89.2%) [8]. These studies showed the usefulness of a conventional side or forward-viewing scope in patients with Billroth II gastrectomy. However, these scopes cannot achieve the afferent loop intubation in 10% of patients due to a long and angulated afferent loop.

Since the introduction of the balloon enteroscope (BE) for small bowel disorders [9], balloon-assisted ERCP, such as single-balloon enteroscopy (SBE)-assisted ERCP, or doubleballoon enteroscopy (DBE)-assisted ERCP, have been developed for patients with SAA. Despite the evident effectiveness of BE-assisted ERCP, it is still more challenging to perform than ERCP in patients with normal anatomy in terms of scope insertion, biliary cannulation, and subsequent diagnostic and interventional procedures, such as forceps biopsy, stone extraction, and stent placement. Recently, interventional endoscopic ultrasound (EUS), such as EUS-guided biliary drainage (EUS-BD) or EUS-guided antegrade intervention, have been available for difficult cases of ERCP, making it a good option for patients with SAA. In this review, we discuss recent advances in interventional ERCP and EUS for patients with SAA.

### **2. Balloon Enteroscope**

Table 1 shows the specifications of the SBE and DBE presently available. The BEs are advanced by holding and shortening the intestine with an inflated balloon. The difference of SBE and DBE is the number of balloons (Figure 1). A balloon is attached to the tip of the over-tube for SBE. DBE equips two balloons. One is attached to the tip of the endoscope while another is attached to the tip of the over-tube. Moreover, the working channel port in SBE appears in an 8 o'clock direction on the endoscopic screen. In contrast, it shows in a 5:30 o'clock direction for DBE.


**Table 1.** Specifications of single-balloon enteroscopy (SBE) and double-balloon enteroscopy (DBE).

SBE, single-balloon enteroscopy. DBE, double-balloon enteroscopy.

**Figure 1.** Balloon enteroscope: (**a**) double-balloon enteroscopy and (**b**) single-balloon enteroscopy.

Use of conventional SBE and DBE is limited by their long working length of 200 cm. Therefore, only a few ERCP accessories are available. Recently, a short-type SBE (short SBE) and DBE (short DBE) with a working length of 152 cm (short SBE) and 155 cm (short DBE), and with a working channel diameter of 3.2 mm is available to increase accessories that can be used for BE-assisted ERCP. Moreover, the short SBE permits the function of passive bending and high-force transmission [10], and the short DBE permits the function of adaptive bending and advanced force transmission [11]. When using SBE, if the scope is at the intestinal tract wall when passing through a sharp flexure, then the passive bending section allows the scope to smoothly bend along the bend of the wall, making it possible to move forward. High-force transmission capabilities make it possible to perform torque operations efficiently and to provide better scope control. Therefore, it is also useful for bile duct cannulation and subsequent treatment procedures. In short, DBE, adaptive bending, and advanced force transmission provide a similar role to passive bending and high-force transmission. These features have contributed to overcoming the difficulties of scope insertion to the target site or biliary cannulation.

In general, ERCP-related procedures using BE are performed under conscious sedation, such as intravenous midazolam and pethidine. During scope insertion, patients are positioned in the prone position. However, for difficult cases, the position may be changed or abdominal pressure may be used. In case the BE forms a loop during insertion, the small intestine is fixed using the inflated balloon and shortened by withdrawing the BE. It is useful and safe for scope insertion to use carbon dioxide. In some difficult cases, such as long afferent limbs seen in Roux-en-Y reconstruction cases, it is difficult to proceed to the target site using short BE. Hence, a change to a conventional-type enteroscope (working length of 200 cm) is required [12]. A transparent hood is useful not only for scope insertion but also for subsequent procedures, such as biliary cannulation [13]. Since postoperative adhesions tend to occur in patients with SAA, endoscopists could feel adhesions during scope insertion or shortening. It must be taken into consideration that there is an increased risk of perforation during scope insertion in patients with SAA than in anatomically normal cases. After achievement of scope insertion to the target site, biliary cannulation is performed using a catheter with a guidewire for cholangiography and deep cannulation. After biliary cannulation, endoscopic diagnosis/interventions, such as stone extraction, stent placement, and biopsy/cytology for diagnosis are performed.

Although endoscopic sphincterotomy (EST) is one of the common procedures in ERCP, it can be particularly troublesome in patients with SAA (Billroth II gastrectomy or Roux-en-Y gastrectomy). It is considered to be difficult because the correct direction of the incision is sometimes uncertain due to the upside-down position in these patients. If the incision is made in the wrong direction, perforation could occur. One study from

a tertiary referral endoscopy center evaluated 40 cases of the endoscopic papillary large balloon (over 10-mm) dilation (EPLBD) without EST for stone extraction in patients with Billroth II gastrectomy. Stones were successfully removed in all cases. Acute complications from EPLBD included mild pancreatitis in two patients (5.0%) [14]. This result showed the usefulness and safety of EPLBD without EST. If an endoscopist feels difficult to perform EST in patients with Billroth II gastrectomy or Roux-en-Y gastrectomy, EPLBD without EST may be recommended.

### **3. Single Balloon-Assisted ERCP**

Table 2 shows outcomes of SBE-assisted ERCP procedures in patients with SAA [12,13,15–22]. The latest systematic review and meta-analysis reported that the pooled data reaching the target site, biliary cannulation, and procedural success rates were 86.6%, 90%, and 75.8%. Adverse events occurred in 6.6% of the procedures [23]. Fatal pancreatitis and intestinal perforation requiring surgical operation were included in the report. Although these were acceptable adverse event rates, we must be mindful that fatal adverse events can occur. It was also reported that bilateral stenting (partial stent-in-stent placement method) using self-expandable metallic stents for patients with hilar bile duct cancer was possible by use of short SBE [24].



SBE, single-balloon enteroscopy. ERCP, endoscopic retrograde cholangiopancreatography. SAA, surgically altered anatomy. N/A, not available.

> Selective biliary cannulation seems to be more difficult in patients with SAA than patients with normal anatomy. The reason is the following: the papilla appears inverted, the view of the papilla tends to be tangential, SBE is forward-viewing, and the elevator system is not equipped. There are several tips for biliary cannulation using SBE. As previously mentioned, the use of a transparent hood is effective for biliary cannulation [13]. Moreover, it was reported that suction of the papilla into the transparent cap facilitated biliary cannulation [25]. The retroflex position contributes to gaining a better view of the papilla in patients with Roux-en-Y gastrectomy. [20,26]. To achieve the retroflex position, the endoscope is advanced while using the upper angle at the inferior duodenal angle. The scope provides a J-turn form (Figure 2). Moreover, cannulation techniques, such as the double-guidewire method, insertion along the pancreatic duct (PD) stent [27], and use of the unique cannula equipped double-lumen [28] are useful.

**Figure 2.** Retroflex position: (**a**,**c**). The papilla is positioned tangential, so it is difficult for biliary cannulation. (**b**,**d**) The endoscope is advanced while using the up angle at the inferior duodenal angle. As a result, it provides a better view of the papilla.

Some studies have reported factors affecting procedural results. One study reported that pancreatic indications, first ERCP attempt, and no transparent hood affected procedural failure [21]. Another study reported that malignant biliary obstruction, first ERCP attempt, and Roux-en-Y reconstruction affected procedural failure [12]. Figure 3 demonstrates endoscopic stone extraction using short SBE for patients with SAA.

**Figure 3.** Endoscopic stone extraction using short single-balloon enteroscopy (short SBE) for patients with surgically altered anatomy (SAA): (**a**) Cholangiography showing a 15-mm biliary stone (pink arrow) in the distal bile duct. (**b**,**c**) Endoscopic papillary large balloon dilation was performed for stone extraction. The balloon was inflated up to 13-mm. (**d**) Stone extraction was completed without crushing.

### **4. Double Balloon-Assisted ERCP**

Table 3 shows the outcomes of DBE-assisted ERCP procedures in patients with SAA [29–38]. The latest systematic review and meta-analysis reported that the pooled data reaching the target site, biliary cannulation, and procedural success rates were 90%, 94%, and 93%. Adverse events occurred in 4% [39]. One case of intestinal perforation requiring surgery was included in the report. A single-center large cohort study reported that Billroth II gastrectomy (B-II) and the native papilla were notable risk factors for complications [40]. In that report, especially cases of B-II with an extremely short afferent loop between the gastro-jejunal anastomosis and Treitz ligament, had a risk of perforation because B-II with

an extremely short afferent loop tend to receive a strong force while proceeding a scope into the afferent loop. This kind of perforation could also occur in SBE.

**Table 3.** Outcomes of double balloon endoscopy (DBE)-assisted ERCP procedure in patients with surgically altered anatomy (SAA).


DBE, double-balloon enteroscopy. ERCP, endoscopic retrograde cholangiopancreatography. SAA, surgically altered anatomy. N/A, not available.

> There are several technical tips for DBE. As previously mentioned, the retroflex position is also useful for biliary cannulation using DBE. Since the working channel port shows up in a 5:30 o'clock direction on the endoscopic screen, positioning and fixing the papilla in a 6 o'clock direction is effective to perform endoscopic sphincterotomy safely [41]. This position provides the oral protrusion and the hooding fold, which are landmarks of the direction of bile duct in performing endoscopic sphincterotomy. Furthermore, it enables confirmation whether common bile duct stones are present or not between the balloon and common bile duct during endoscopic papillary large balloon dilation [42].

> Factors affecting procedural results using DBE have also been reported. One study noted that patients with surgery during childhood, biliary atresia, and second operation post-transplant were factors affecting procedure results in patients with Roux-en-Y reconstruction [36]. Another study reported that Roux-en-Y reconstruction and the first-time procedure affected the outcomes and adverse events [38]. In the report, a physician in training did not significantly affect the outcomes.

### **5. Other Device-Assisted ERCP**

There are several reports of ERCP using other devices. Motorized spiral enteroscopy (PSF-1, Olympus Medical Systems, Tokyo, Japan) with a working length of 168 cm, and with a working channel diameter of 3.2 mm is available from 2015. The drive motor located in the endoscope handle is activated by foot pedals and controls the direction and speed of rotation of a coupler located in the middle of the endoscope's insertion tube. The single-use spiral assembly is composed of corrugated tubing with an atraumatic plastic spiral bonded to its exterior. It relies on rotation of the spiral component to "pleat" or "un-pleat" the bowel either on or off the insertion tube as the spiral thread rotates in a clockwise or counterclockwise direction, respectively [43–45]. It has been evaluated in prospective clinical trials and shown to be safe and effective for deep enteroscopy [45]. Moreover, in view of ERCP, it allows the uses of standard ERCP-accessories in the same way as short SBE and DBE. Actually, there is one report published regarding motorized spiral enteroscopy-assisted ERCP in a patient with SAA, showing successful and rapid enteroscopic access, cannulation, and balloon dilation therapy [46]. Although further studies are needed, it could be the upcoming ERCP technology in pa-tients with SAA.

Moreover, laparoscopy-assisted ERCP (LA-ERCP) is accomplished by placing a trocar in the remnant stomach under laparoscopic guidance followed by insertion of the conventional duodenoscope through the trocar to reach the papilla. ERCP is then carried out in a standard method. The advantage of LA-ERCP is that the duodenoscope, which is used for ERCP when normal anatomy is available. It was reported that LA-ERCP achieved high success rates [47,48]. A multicenter study reported that the procedural success, and adverse events rates were 98%, and 18% (laparoscopy related, 10%, ERCP related, 7%, both, 1%) [49]. Although there is a high success rate, the overall adverse event rate was high due to the added laparoscopy-related events.

### **6. Interventional EUS**

Despite the high effectiveness reported for BE-assisted ERCP in patients with SAA, it has several challenges for successful completion of procedures. Alternative treatment modalities are needed for some cases. Percutaneous transhepatic biliary drainage (PTBD) has been traditionally performed in these patients despite PTBD being associated with a higher adverse event rate than ERCP [50]. PTBD is conventionally performed using the following three-step approach: (1) external drainage with confirmation of clinical improvement, (2) stent deployment with maintenance of the external drainage tube, and (3) external drainage tube removal after the confirmation of proper drainage through the stent. Although PTBD is one of the alternatives, it may be impractical for urgent cases due to the requirement of serial dilation and track maturation [51]. Moreover, external drainage tube trouble could be caused. However, PTBD is possible to perform stone extraction effectively and safely, so we can choose PTBD as the alternatives for cases of difficult stone extraction using BE.

Recently, interventional EUS has been in the spotlight as an alternative therapy for patients with difficult ERCP, such as scope insertion and biliary cannulation. Interventional EUS may be a first-line treatment in some cases, such as malignant cases with cancer invasion of the small intestines or papilla [12].

There are several drainage methods for interventional EUS [52]. The first method is the EUS-guided hepaticogastrostomy (EUS-HGS). Generally, the left intrahepatic bile duct (B 2 or 3) is punctured to make the drainage route. After cholangiography and guidewire insertion, the fistula is dilated using a dilation device followed by the placement of a biliary stent [53]. If the stomach has been resected, such as in Roux-en-Y gastrectomy cases, a puncture is performed from the jejunal limb. The second method is EUS-guided antegrade stenting (EUS-AG). After puncture of the left intrahepatic bile duct, a guidewire is directed to the papilla or hepaticojejunal anastomosis, and the biliary stent is placed via an antegrade route [54]. Moreover, the EUS-guided rendezvous technique (EUS-RV) is also a useful alternative procedure [55]. In cases of difficult biliary cannulation using a BE, after the left intrahepatic bile duct (B2 or B3) is punctured, the guidewire is directed beyond the papilla or hepaticojejunal anastomosis. As a result, the guidewire is positioned into the duodenum or jejunum. Afterward, a scope exchange from the echoendoscope to BE is carried out. The guidewire is grasped using a forceps device and pulled into the working channel. Finally, biliary cannulation through the papilla or anastomotic site is successful.

Table 4 shows outcomes of EUS-guided biliary drainage (EUS-BD) [56–64]. The latest systematic review and meta-analysis reported that the pooled technical success rates and clinical success rates were 91.5% and 87%, respectively. Adverse events occurred in 17.9%. The main adverse events were bile leakage (4.1%), stent migration (3.9%), and infections (3.8%) [65]. Although there were high success rates using interventional EUS, adverse events were higher than BE-assisted ERCP. Therefore, EUS-BD should be performed carefully and endoscopists should take into consideration that severe adverse events could develop. Figure 4 provides the successful EUS-HGS in a patient with SAA. Although SBE-assisted ERCP was initially performed, it failed due to cancer invasion of the small intestine.


**Table 4.** Outcomes of endoscopic ultrasound (EUS)-guided biliary drainage.

EUS, endoscopic ultrasound. N/A, not available.

**Figure 4.** Endoscopic ultrasound-guided hepaticogastrostomy for patients with surgically altered anatomy (SAA) showing a failed case of single-balloon enteroscopy-assisted (SBE) endoscopic retrograde cholangiopancreatography (ERCP). (**a**) It was impossible to reach the papilla due to cancer invasion of the duodenum. (**b**) Fluoroscopic image showing duodenal obstruction due to cancer invasion (pink arrow). (**c**) Endoscopic ultrasound-guided hepaticogastrostomy is performed. First, B 3 is punctured using a 19-gauge needle. After puncture, we performed cholangiography to confirm the position of the guidewire. (**d**) Finally, a biliary stent was placed.

#### **7. Comparison between BE-Assisted ERCP and Interventional EUS**

Some papers have conducted a comparison between BE-assisted ERCP and EUS-BD in patients with SAA. A multicenter retrospective study reported that clinical success was 88% in the EUS-BD group. It was 59.1% in the BE-assisted ERCP group (odds ratio [OR] 2.83, *p* = 0.03). The EUS-BD group completed the procedure in a shorter amount of time than the BE-assisted ERCP group (55 min vs. 95 min, *p* < 0.0001). However, adverse events occurred more often in the EUS-BD group (20% vs. 4%, *p* = 0.01) [66]. An international multicenter study compared EUS-BD and BE-assisted ERCP in patients with Roux-en-Y gastric bypass and showed that the technical success rate of EUS-BD was superior to BE-assisted ERCP (100% vs. 60%). Adverse events occurred comparably [67]. These comparison studies had lower success rates than studies in Tables 2 and 3. These comparison studies' population were almost all R-Y reconstruction. Studies in Tables 2 and 3 included Billroth II gastrectomy and pancreaticoduodenectomy, which are considered to be easier than R-Y. Therefore, these success rates for BE would be lower than Tables 2 and 3.

Although interventional EUS provided a higher success rate and shorter procedure time, adverse events tended to be high. A fatal complication, such as aberrant stent displacement into the abdominal cavity, has been reported [68]. Dedicated devices used by EUS-BD are warranted for safety. Hence, the choice between BE-assisted ERCP and interventional EUS depends on the postoperative reconstruction, patient's condition, or the expertise of the endoscopist.

### **8. Conclusions**

We discussed recent advances in interventional ERCP and EUS for patients with SAA. Both BE-assisted ERCP and interventional EUS have advantages and disadvantages. The choice of procedure should be individualized to the patient's condition or the expertise of the endoscopist. We propose the following interventional strategy for patients with SAA (Figure 5). First, if tumor invasion to the small intestine can be adequately predicted prior to the procedure by cross-sectional imaging, such as computed tomography, the most appropriate technique for the case, such as PTBD or EUS-BD, can be selected as alternative interventions. During the procedure, if the target site (papilla or hepaticojejunal anastomosis) cannot be reached using a BE, laparoscopy-assisted ERCP, PTBD, or EUS-BD will be required to complete the treatment procedure. In case of failed biliary cannulation or an intended procedure, reattempting BE-assisted ERCP, PTBD, or EUS-BD should be selected according to the previous treatment.

**Figure 5.** Flowchart of our proposed interventional strategy for patients with surgically altered anatomy (SAA). ERCP, endoscopic retrograde cholangiopancreatography. EUS-BD, endoscopic ultrasound-guided biliary drainage. PTBD, percutaneous transhepatic biliary drainage. LA-ERCP, laparoscopy-assisted ERCP. BE-ERCP, balloon enteroscope-assisted ERCP.

Further improvement of both BE-assisted ERCP and interventional EUS are needed to perform effective and safe procedures for patients with SAA.

**Author Contributions:** The paper was authored by Y.T., who designed and drafted the article. M.M., A.F., T.O., M.S., H.K., Y.S., K.M., T.T., Y.M., and S.R. provided a critical revision of the article for important intellectual content. Y.T. finally approved the article for submission. The final version of the manuscript was approved by all authors. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


## *Review* **Chemotherapy for Biliary Tract Cancer in 2021**

**Takashi Sasaki \*, Tsuyoshi Takeda, Takeshi Okamoto, Masato Ozaka and Naoki Sasahira**

Department of Hepato-Biliary-Pancreatic Medicine, Cancer Institute Hospital of Japanese Foundation for Cancer Research, 3-8-31, Ariake, Koto-ku, Tokyo 135-8550, Japan; tsuyoshi.takeda@jfcr.or.jp (T.T.); takeshi.okamoto@jfcr.or.jp (T.O.); masato.ozaka@jfcr.or.jp (M.O.); naoki.sasahira@jfcr.or.jp (N.S.)

**\*** Correspondence: sasakit-tky@umin.ac.jp; Tel.: +81-3-3520-0111; Fax: +81-3-3520-0141

**Abstract:** Biliary tract cancer refers to a group of malignancies including cholangiocarcinoma, gallbladder cancer, and ampullary cancer. While surgical resection is considered the only curative treatment, postoperative recurrence can sometimes occur. Adjuvant chemotherapy is used to prolong prognosis in some cases. Many unresectable cases are also treated with chemotherapy. Therefore, systemic chemotherapy is widely introduced for the treatment of biliary tract cancer. Evidence on chemotherapy for biliary tract cancer is recently on the increase. Combination chemotherapy with gemcitabine and cisplatin is currently the standard of care for first-line chemotherapy in advanced cases. Recently, FOLFOX also demonstrated efficacy as a second-line treatment. In addition, efficacies of isocitrate dehydrogenase inhibitors and fibroblast growth factor receptor inhibitors have been shown. In the adjuvant setting, capecitabine monotherapy has become the standard of care in Western countries. In addition to conventional cytotoxic agents, molecular-targeted agents and immunotherapy have been evaluated in multiple clinical trials. Genetic testing is used to check for genetic alterations and molecular-targeted agents and immunotherapy are introduced based on tumor characteristics. In this article, we review the latest evidence of chemotherapy for biliary tract cancer.

**Keywords:** biliary tract cancer; cholangiocarcinoma; chemotherapy; cytotoxic agents; molecular targeted agents; immunotherapy; precision medicine; genetic testing

### **1. Introduction**

Biliary tract cancer is a heterogeneous group of highly aggressive cancers including intrahepatic/perihilar/distal cholangiocarcinoma, gallbladder cancer, and ampullary cancer [1]. Biliary tract cancer is common in Japan, Southeast Asia, South America, and India [2,3]. Cholangiocarcinoma has been increasing worldwide, while the incidence of gallbladder cancer has been decreasing in recent years [4–6]. In Japan, the incidence and mortality of biliary tract cancer have plateaued over the last decade, with an annual incidence and mortality of approximately 22,000 and 18,000, respectively [7]. This cancer is still the sixth leading cause of cancer-related death. In Japan, more than 45% of new cases are diagnosed over the age of 80.

While surgical resection is considered the only curative treatment, postoperative recurrence can sometimes occur. Data from the biliary tract cancer registry in Japan revealed that five-year survival rates were 39.8% for gallbladder cancer, 24.2% for perihilar cholangiocarcinoma, 39.1% for distal cholangiocarcinoma, and 61.3% for ampullary cancer [8]. Adjuvant chemotherapy is sometimes introduced to achieve long-term survival for resected cases with poor prognostic factors. Many unresectable cases are also treated with chemotherapy. As surgery for biliary tract cancer is a highly invasive procedure, surgery may be avoided in potentially resectable cases due to old age or comorbidities. Therefore, systemic chemotherapy is widely introduced for the treatment of biliary tract cancer. Recently, evidence on chemotherapy for biliary tract cancer is on the increase. In addition to conventional cytotoxic agents, molecular-targeted agents and immunotherapy

**Citation:** Sasaki, T.; Takeda, T.; Okamoto, T.; Ozaka, M.; Sasahira, N. Chemotherapy for Biliary Tract Cancer in 2021. *J. Clin. Med.* **2021**, *10*, 3108. https://doi.org/10.3390/ jcm10143108

Academic Editor: Stanley W. Ashley

Received: 18 June 2021 Accepted: 13 July 2021 Published: 14 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

have widely been introduced in this field. Genetic testing is used to check for genetic alterations and molecular-targeted agents and immunotherapy are introduced based on tumor characteristics. Here, we review the latest evidence on chemotherapy for biliary tract cancer.

### **2. First-Line Chemotherapy for Advanced Biliary Tract Cancer**

Standard chemotherapy for biliary tract cancer was not established until about 2000. Until then, chemotherapy for pancreatic cancer had been used as a reference. The efficacy of chemotherapy was confirmed in a randomized control study conducted before 2000 which compared chemotherapy to best supportive care in advanced pancreatic and biliary tract cancers [9]. Subsequently, a randomized controlled study comparing chemotherapy and best supportive care for unresectable gallbladder cancer was reported from India in 2010, confirming the usefulness of chemotherapy [10]. Between 2000 and 2010, gemcitabine and 5-fluorouracil were considered the key drugs for the treatment of advanced cases. A pooled analysis of clinical trials conducted between 1985 and 2006 identified gemcitabine, fluoropyrimidines, and cisplatin as the key active agents and concluded that gemcitabine combined with platinum compounds represented the provisional standard of chemotherapy for advanced biliary tract cancer [11].

The combination chemotherapy of gemcitabine and platinum compounds demonstrated good efficacy in advanced cases. A randomized phase II study (ABC-01) comparing the doublet of gemcitabine and cisplatin to gemcitabine alone was reported from the United Kingdom [12]. The doublet regimen was associated with improved tumor control and progression-free survival. Based on this result, the study was extended to a phase III study (ABC-02) to verify the prognostic effect of the combination chemotherapy relative to gemcitabine monotherapy [13]. Four hundred ten patients were randomized to receive either gemcitabine and cisplatin combination chemotherapy or gemcitabine alone. The primary endpoint was overall survival. The median overall survival was 11.7 months in the combination group and 8.1 months in the monotherapy group (hazard ratio, 0.64; *p <* 0.001). The median progression-free survivals of the combination and monotherapy groups were 8.0 months and 5.0 months, respectively (*p <* 0.001). The rate of tumor control among patients in the combination group was significantly increased (81.4% vs. 71.8%, *p =* 0.049). Although neutropenia occurred more frequently in the combination group, combination chemotherapy with gemcitabine and cisplatin was considered a feasible regimen for advanced biliary tract cancer. This combination chemotherapy was also evaluated in Japanese patients and similar efficacy was confirmed in a multicenter, randomized phase II study (BT-22) [14]. Treatment was repeated for up to 24 weeks in the ABC-02 study and up to 48 weeks in the BT-22 study. In a meta-analysis of these two studies, the efficacy of gemcitabine and cisplatin combination chemotherapy was confirmed in patients with good performance status (performance status of 0 or 1) and in patients with cholangiocarcinoma or gallbladder cancer [15]. On the other hand, the superiority of this combination chemotherapy was not shown in patients with poor performance status or ampullary cancer. The major grade 3/4 adverse events of gemcitabine and cisplatin combination chemotherapy were neutropenia and anemia. We also need to pay attention to renal dysfunction and hearing loss. Oxaliplatin is another platinum compound known to cause less renal damage and therefore does not require aggressive hydration, unlike cisplatin. Oxaliplatin is sometimes used as a substitute for cisplatin. However, the noninferiority of gemcitabine and oxaliplatin combination chemotherapy, when compared to gemcitabine and cisplatin combination chemotherapy, has not been proven. One randomized controlled study comparing these two regimens was conducted in India [16]. A total of 243 patients with unresectable gallbladder cancer were randomly assigned to one of these two regimens. The median overall survivals of gemcitabine and oxaliplatin combination chemotherapy and gemcitabine and cisplatin combination chemotherapy were 9.0 months and 8.3 months, respectively (hazard ratio, 0.78; *p =* 0.057). Because the predetermined statistical threshold was not met, the study failed to prove non-inferiority. Moreover, this study was underpowered to determine the superiority of gemcitabine and oxaliplatin combination chemotherapy.

Several randomized controlled studies have been conducted in pursuit of treatment regimens that are superior to the standard treatment of gemcitabine and platinum compounds. Some involved combination chemotherapies which added a third drug to the doublet, while others involved a novel regimen. Table 1 summarizes previous randomized controlled studies on first-line chemotherapy for advanced biliary tract cancer. No additional benefits of epidermal growth factor receptor and vascular endothelial growth factor receptor inhibitors have been observed to date [17]. On the other hand, good results have been obtained with S-1, which is widely used in Japan [18,19].


**Table 1.** Randomized controlled studies on first-line chemotherapy for advanced biliary tract cancer.


**Table 1.** *Cont.*

N; number, RR; response rate, PFS; progression-free survival, OS; overall survival, M; months, rP2; randomized phase II study, GemCis; gemcitabine + cisplatin, GEM; gemcitabine, GEMOX; gemcitabine + oxaliplatin, 5FU; 5-fluorouracil, FA; folinic acid, BSC; best supportive care, CDDP; cisplatin, Cape; capecitabine, mFOLFIRINOX; modified FOLFIRINOX (5-fluorouracil + leucovorin + irinotecan + oxaliplatin), Cmab; cetuximab, Pmab; panitumumab; FOLFOX; 5-fluorouracil + leucovorin + oxaliplatin, LV; leucovorin, CPT-11; irinotecan.

> S-1 is an oral fluoropyrimidine derivative used mainly in Asian countries. The combination of gemcitabine and S-1 was widely evaluated in phase II and randomized phase II studies in Japan [34–37]. Based on these results, a randomized phase III study comparing gemcitabine and S-1 combination chemotherapy with gemcitabine and cisplatin combination chemotherapy was conducted in Japan [18]. This study was conducted to evaluate the non-inferiority of gemcitabine and S-1 combination chemotherapy compared to gemcitabine and cisplatin combination chemotherapy. Patients with advanced biliary tract cancer were randomly assigned either gemcitabine and S-1 combination chemotherapy or gemcitabine and cisplatin combination chemotherapy. The primary endpoint was overall survival. The median overall survivals were 15.1 months and 13.4 months, respectively (hazard ratio 0.945, *p* = 0.046 for non-inferiority). Because the toxicities of gemcitabine and S-1 combination chemotherapy were deemed acceptable, this new doublet also became the standard of care for patients with advanced biliary tract cancer. The major grade 3/4 adverse event of gemcitabine and S-1 combination chemotherapy was neutropenia. We also need to pay attention to diarrhea, oral mucositis, maculopapular rash, and skin hyperpigmentation. S-1 was also evaluated as the triplet with gemcitabine and cisplatin. Based on the good result of a phase II study evaluating the efficacy of gemcitabine + cisplatin + S-1 combination chemotherapy [38], a phase III study was conducted to confirm the superiority of this triplet over gemcitabine and cisplatin combination chemotherapy in Japan [19]. Two hundred forty-six patients with advanced biliary tract cancer were randomized 1:1 to receive either the triplet or the doublet chemotherapy. The primary endpoint was overall survival. The median overall survivals of gemcitabine + cisplatin + S-1 combination chemotherapy and gemcitabine + cisplatin combination chemotherapy were 13.5 months and 12.6 months, respectively (hazard ratio 0.791, *p* = 0.046). This adverse event's profile of the triplet chemotherapy was also acceptable. The major grade 3/4 adverse event of triplet chemotherapy was also neutropenia. This triplet is also needed to pay attention to diarrhea, stomatitis, and rash. Therefore, gemcitabine + cisplatin + S-1 combination chemotherapy is currently considered a standard regimen for advanced cases.

> In summary, the global standard first-line chemotherapy for advanced biliary tract cancer is still gemcitabine and cisplatin combination chemotherapy. In Japan, gemcitabine + S-1 combination chemotherapy and gemcitabine + cisplatin + S-1 combination chemotherapy are also considered alternatives of gemcitabine + cisplatin combination chemotherapy in the first-line setting.

#### **3. Second-Line Chemotherapy for Advanced Biliary Tract Cancer**

The usefulness of second-line chemotherapy has been reported based on a systematic review and large retrospective studies, but standard treatment has not been

established [39–45]. In Japan, S-1 is widely used as monotherapy in the clinical setting [46,47]. To establish the standard treatment of second-line chemotherapy, various treatments such as molecular-targeted agents and immunotherapy are being developed in addition to conventional cytotoxic agents [48]. Recently, several randomized phase II and phase III studies were reported, some of which showed positive results. Table 2 summarizes previous randomized controlled studies of second-line or third-line chemotherapy for advanced biliary tract cancer.


**Table 2.** Randomized controlled studies on second-line or third-line chemotherapy for advanced biliary tract cancer.

N; number, RR; response rate, PFS; progression-free survival, OS; overall survival, M; months, rP2; randomized phase 2 study, BSC; best supportive care, FOLFOX; 5-fluorouracil + leucovorin + oxaliplatin, ASC; active symptom control, Cape; capecitabine, MMC; mitomycin-C, 5FU; 5-fluorouracil, LV; leucovorin, nal-IRI; nano-liposomal irinotecan.

> A phase III study (ABC-06) comparing FOLFOX (5-fluorouracil + leucovorin + oxaliplatin) and active symptom control was conducted in the United Kingdom [50]. Patients with advanced biliary tract cancer treated previously with gemcitabine and cisplatin combination chemotherapy were included. Enrolled patients were randomized to receive either FOLFOX or active symptom control, which was the equivalent of best supportive care. Patients in the active symptom control group could receive FOLFOX after radiographic disease progression was confirmed. The primary endpoint was overall survival. The median overall survivals of FOLFOX and active symptom control groups were 6.2 months and 5.3 months, respectively (hazard ratio 0.69, *p* = 0.031). The benefit of FOLFOX was consistent across subgroups, including those with platinum sensitivity during first-line chemotherapy. The major grade 3/4 adverse events of FOLFOX were neutropenia, fatigue, and catheter-related infection. We also need to pay attention to peripheral neuropathy. This study was the first prospective phase III study that confirmed the benefit of chemotherapy after combination chemotherapy with gemcitabine and cisplatin. Another positive phase III study that showed the efficacy of second-line chemotherapy was the ClarIDHy study. This study was a global phase III study comparing ivosidenib and best supportive care.

Ivosidenib is a first-in-class, oral, targeted, small-molecule inhibitor of mutant isocitrate dehydrogenase (IDH) 1 protein. IDH1 mutations occur in up to 20% of cholangiocarcinomas. Patients with advanced cholangiocarcinoma who had received 1–2 prior therapies were enrolled in this study. Patients were randomly assigned to either the ivosidenib group or the best supportive care group. The primary endpoint was progression-free survival. The median progression-free survivals of the ivosidenib and best supportive care groups were 2.7 months and 1.4 months, respectively (hazard ratio 0.37, *p* < 0.001). The major grade 3/4 adverse events of ivosidenib were reported as ascites. This study was the first prospective phase III study that demonstrated a clinical benefit in targeting a molecularly defined subgroup of cholangiocarcinoma and in evaluating genetic profiles of biliary tract cancer. In 2021, the result of a randomized phase II study (NIFTY) comparing 5-fluorouracil + leucovorin + nano-liposomal irinotecan and 5-fluorouracil + leucovorin was reported [58]. This triplet chemotherapy is now known as the NAPOLI regimen and is widely used for second-line chemotherapy in advanced pancreatic cancer. The additional benefit of nano-liposomal irinotecan was demonstrated in this study. Two other randomized phase II studies also showed positive results with capecitabine and irinotecan combination chemotherapy and with regorafenib monotherapy. However, the number of patients enrolled in these studies was relatively small. Therefore, further evaluation is required to establish more solid evidence on these two regimens.

Biliary tract cancers are a heterogeneous group of cancers with different genetic alteration profiles [59–62]. Potential clinically actionable alterations, defined as oncogenic driver alterations with matched therapeutic agents either under investigation or approved in other tumor types, were identified in 44.5% of patients, showing promise for precision medicine in this field [62]. Common genes implicated in biliary tract cancer tumorigenesis include IDH1, IDH2, fibroblast growth factor receptor (FGFR) 1, FGFR2, FGFR3, and human epidermal growth factor receptor (HER) 2. Encouraging results were seen in patients with identified mutational targets, especially in tumors harboring FGFR2 fusions, HER2, and IDH mutations. The efficacy of an IDH1 inhibitor (ivosidenib) was shown in a phase III study [49]. Several FGFR inhibitors have been evaluated in phase II studies [63–66]. FGFR2 rearrangements were reported in 7.4% and 3.6% of Japanese intrahepatic cholangiocarcinoma and perihilar cholangiocarcinoma patients, respectively [67]. Based on the results of a phase II study (FIGHT-202) [65], pemigatinib was approved in many countries for patients with FGFR2 fusion or rearrangement. The major grade 3/4 adverse events of pemigatinib were hypophosphatemia, arthralgia, stomatitis, hyponatremia, abdominal pain, and fatigue.

The efficacies of pembrolizumab for microsatellite instability (MSI)-high solid tumors [68] and neurotrophic tyrosine receptor kinase (NTRK) inhibitors (entrectinib and larotrectinib) for solid tumors with NTRK fusion have also been reported [69,70]. Only a few biliary tract cancer patients were included in these studies, owing to the rarity of these alterations. MSI-high biliary tract cancer was reported in 2.22% and 1.50% of Japanese cholangiocarcinoma and gallbladder cancer patients, respectively [71]. NTRK fusion positivity was reported in only 0.18% of biliary tract cancers [72]. The efficacy and safety of pembrolizumab were evaluated in KEYNOTE-028 and KEYNOTE-158 [73]. Pembrolizumab provides durable antitumor activity in 6–13% of patients with advanced biliary tract cancer regardless of programmed cell death 1 ligand 1 (PD-L1) expression and has manageable toxicity. Other immune checkpoint inhibitors were also evaluated in phase I or II studies involving both naïve and refractory advanced biliary tract cancer [74–79]. The results of these studies were promising, and further large-scale evaluation is underway. When using these immune checkpoint inhibitors, appropriate management of immune-related adverse events is required.

In summary, FOLFOX is becoming the standard second-line chemotherapy for refractory cases. The presence of IDH mutations, FGFR fusion/rearrangement and NTRK fusion, as well as MSI status, should be confirmed to consider treatment with relevant inhibitors or immune checkpoint inhibitors where applicable. It is also important to consider participation in clinical studies if molecular-targeted agents matched with identified gene alterations are available.

### **4. Adjuvant Chemotherapy for Resected Biliary Tract Cancer**

While surgical resection is regarded as the only treatment with a chance of curing biliary tract cancer, postoperative recurrence can sometimes occur. However, standard adjuvant chemotherapy has not been established to date.

Several phase III studies have been reported on adjuvant chemotherapy for resected biliary tract cancer. The first phase III study evaluated the efficacy of adjuvant chemotherapy of 5-fluorouracil + mitomycin-C versus surgery alone in patients with resected pancreaticobiliary carcinoma [80]. Results indicated that gallbladder carcinoma patients who underwent noncurative resection may derive some benefit from systemic chemotherapy. However, alternative modalities must be developed for patients with carcinomas of the pancreas, bile duct, or ampulla of Vater. Several prospective phase III studies focused on adjuvant chemotherapy for biliary tract cancer were subsequently conducted, as summarized in Table 3.

**Table 3.** Randomized controlled studies of adjuvant chemotherapy for resected biliary tract cancer.


N; number, RFS; recurrent-free survival, OS; overall survival, M; months, EHCC; extrahepatic cholangiocarcinoma, AC; ampullary cancer, ICC; intrahepatic cholangiocarcinoma, GBC; gallbladder cancer, 5FU; 5-fluorouracil, FA; folinic acid, GEM; gemcitabine, GEMOX; gemcitabine + oxaliplatin.

> ESPAC-3 was a phase III study that evaluated the efficacy of adjuvant chemotherapy using 5-fluorouracil + folinic acid or gemcitabine monotherapy against surgery alone [81]. Patients with extrahepatic cholangiocarcinoma and ampullary cancer were enrolled in this study. This study did not show superiority of adjuvant chemotherapy over surgery alone based on an intention-to-treat analysis. However, sensitivity analysis adjusted for prognostic factors showed improved prognosis in both the adjuvant chemotherapy group and the gemcitabine monotherapy group compared to the surgery alone group. BCAT was a phase III study conducted to evaluate the efficacy of adjuvant chemotherapy using gemcitabine against surgery alone [82]. This Japanese study was limited to extrahepatic cholangiocarcinoma patients. Treatment outcomes of surgery alone were extremely good, and no additional benefits of gemcitabine were observed. PRODIGE 12-ACCORD 18 was a French phase III study that compared adjuvant gemcitabine and oxaliplatin combination chemotherapy with surgery alone [83]. All types of biliary tract cancer other than ampullary cancer were included. The efficacy of adjuvant combination chemotherapy was not demonstrated in this negative study. BILCAP was a British phase III study that compared adjuvant capecitabine and surgery alone [84]. While capecitabine monotherapy failed to show improvement based on an intention-to-treat analysis, significant improvement was demonstrated in a per-protocol analysis. The major grade 3/4 adverse events of capecitabine were hand-foot syndrome, diarrhea and fatigue. Because of this promising result, the American Society of Clinical Oncology guideline recommends adjuvant capecitabine monotherapy for resected biliary tract cancer [85].

In summary, capecitabine monotherapy of six months for adjuvant chemotherapy is considered standard treatment for resected biliary tract cancer in Western counties. Until prospective studies show otherwise, surgery alone remains the standard of care in Japan.

### **5. Ongoing Clinical Trials for Biliary Tract Cancer**

Currently, effective chemotherapy for biliary tract cancer is extremely limited, and the development of new therapies is urgently needed. There are a large number of ongoing prospective studies for biliary tract cancer [86–91]. Based on promising early-phase study results, phase III studies are underway [92–94]. A list of major ongoing randomized controlled studies for biliary tract cancer is provided in Table 4. In addition to conventional treatments using cytotoxic agents, a wide variety of drugs such as molecular-targeted agents and immune checkpoint inhibitors are being investigated. Despite the low frequency of genetic alterations, precision medicine with molecular-targeted agents holds promise for selected patients. Umbrella and basket studies are increasingly being conducted, based on the need to build a mechanism to provide drugs suited to each genetic alteration regardless of tumor origin. The efficacy of immunotherapy combined with conventional treatment is also being investigated. In addition, a new large-scale trial for neoadjuvant chemotherapy is underway. Many new therapies that enhance the effectiveness of current regimens have been validated in late-phase clinical trials such as those listed in Table 4. On the other hand, many new drugs have been validated in other, slightly earlier phase clinical trials. It is hoped that such drugs will advance to late-phase clinical trials sooner. Like other cancers, it is also expected that molecular-targeted drugs and immunotherapy that matched cancer genetic characteristics, such as first-line FGFR inhibitors, can produce much better treatment than current standard treatments.

**Table 4.** Major ongoing clinical studies for biliary tract cancer.


CDDP; cisplatin, GemCis; gemcitabine + cisplatin, GEMOX; gemcitabine + oxaliplatin, Cape; capecitabine, GEM; gemcitabine, CTx; chemotherapy, GCS; gemcitabine + cisplatin + S-1.

### **6. Conclusions**

Figure 1 shows the proposed treatment algorithm of chemotherapy for advanced biliary tract cancer in 2021. It is necessary to arrange this algorithm according to the medical situation in each country.

**Figure 1.** Proposed treatment algorithm of chemotherapy for advanced biliary tract cancer. GEM; gemcitabine, CDDP; cisplatin, FOLFOX; 5-fluorouracil + leucovorin + oxaliplatin, IDH; isocitrate dehydrogenase, FGFR; fibroblast growth factor receptor, MSI; microsatellite instability, NTRK; neurotrophic tyrosine receptor kinase.

> Biliary tract cancer is considered a population with various genetic alterations. Genetic alterations are often measured before starting second- or third-line chemotherapy only in patients who are able to get enough tissue samples. If the effectiveness of moleculartargeted drugs and immunotherapy based on the characteristics of cancer is shown at first-line setting, it is thought that the trend of investigating genetic alterations from the time of diagnosis will accelerate in the future. In addition, to overcome the problem that biliary tract cancer is sometimes difficult to get enough tissue samples, there are great expectations for liquid biopsy in this field. Furthermore, there is an urgent need to develop more drugs that match genetic alterations and establish a system to deliver the drugs to the matched patients in clinical practice.

> While evidence relating to chemotherapy for biliary tract cancer had been limited, numerous clinical studies have been conducted in the last decade and evidence is steadily accumulating. Many large-scale clinical studies are still underway, some of which may lead to improved treatment outcomes going forward.

> **Author Contributions:** Writing—original draft preparation, T.S.; writing—review and editing, T.T., T.O., supervision, M.O., N.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** T.S. has received honoraria from Taiho Pharmaceutical Co., Ltd., Yakult Honsha Co., Ltd., Eisai Co., Ltd. T.T. has received honoraria from Taiho Pharmaceutical Co., Ltd. M.O. has received honoraria from Taiho Pharmaceutical Co., Ltd., Yakult Honsha Co., Ltd., Eisai Co., Ltd., AstraZeneca, ONO Pharmaceutical Co., Ltd., Chugai Pharmaceutical Co., Novartis, MSD. N.S. has received research grants from Taiho Pharmaceutical Co., Ltd., Chugai Pharmaceutical Co., Ltd., and

has received honoraria Chugai Pharmaceutical Co., Ltd., Eisai Co., Ltd. The other author declares no conflicts of interest.

### **References**

