*2.3. HF10*

HF10 (Canerpaturev, C-REV by Takara Bio Inc. Mountain View, CA, USA) is a HSV-1 strain with a deletion in the Bam HI-B fragment [16,27,28] and additional alterations resulting in defective expression of UL43, UL49.5, UL55, UL56, LAT genes, and increased expression of UL53 and UL54 [17]. In contrast to other oHSVs, HF10 was not engineered—the mutations that define this strain occurred spontaneously [17]. Preclinical evaluation of this construct presented promising results in a syngeneic immunocompetent mouse model for peritoneally disseminated fibrosarcoma with the HF-10-treated animals showing prolonged survival. The development of anti-tumor immunity was also shown in the mice since they rejected a tumor rechallenge [16]. HF10 was first tested in humans in a pilot study to assess toxicity and efficacy in patients with recurrent metastatic breast cancer and (sub)cutaneous metastases [29,30]. One nodule per patient was injected with HF10 for up to three days, while another was injected with saline. No macroscopic reduction of tumors was observed, but histological analysis showed 30–100% tumor cell death and signs of viral infection of breast cancer cells. No shedding or reactivation of HSV-1 was detected. There were no therapy-related adverse effects. A follow-up phase I dose-escalation study examined possible toxicity and efficacy of HF10 in patients with non-resectable pancreatic cancer [31]. HF10 was injected intratumorally at several locations during laparotomy and via catheter for three days in a row. The primary endpoints were assessed 30 days after virus inoculation. No adverse events were registered and approximately 66% of the patients presented with stable disease or even partial response. Furthermore, the tumor marker CA19-9 (cancer antigen 19.9) decreased in 50% of the patients. All of the patients were HSV1 antibody positive from the beginning and no virus shedding could be detected, either in the abdomen or in the blood. Histopathological analysis found scar tissue at the HF10 injection site with virus-specific patterns (inclusion bodies, small segmented nuclei), corresponding with the results of the previous study conducted for breast cancer, suggesting viral replication [30,31]. In comparison to normal tumor tissue, HF10-injected tumors showed a significantly higher rate of CD8+-T-cell and macrophage infiltration. A follow-up phase I study combined ultrasound guided HF-10 injections with erlotinib and gemcitabine chemotherapy in unresectable locally advanced pancreatic cancer [32]. After an initial chemotherapy cycle, patients received intratumoral endoscopic ultrasound (EUS)-guided HF10 injections every two weeks with a total of four injections. While a chemotherapy-related grade III myelosuppression was noticed in 50% of the patients, no HF10-specific adverse events occurred. 90% of the patients received all planned treatments and were assessed for therapy response in accordance with RECIST criteria, with >70% of the patients showing either stable disease or partial response overall. Analysis of target lesion response even showed a partial response in 33% and a stable disease in 66% of the cases. A complete surgical response was noted in two patients who underwent surgery after therapy. An infiltration of CD8<sup>+</sup> T cells was observed in the resected tissue from both patients. Another small pilot study conducted by Fujimoto et al. [33] investigated the effects HF10 in subcutaneous metastases of head and neck squamous cell carcinoma in two patients; the authors admittedly described no macroscopic changes two weeks after virus inoculation, but report tumor cell death and fibrosis as well as an enrichment of CD4+- and CD8+-T-cells in the histopathological analyses of resected tumor specimens.
