*Article* **Quantitative Structure-Activity Relationship of Enhancers of Licochalcone A and Glabridin Release and Permeation Enhancement from Carbomer Hydrogel**

**Zhuxian Wang † , Yaqi Xue † , Zhaoming Zhu, Yi Hu, Quanfu Zeng, Yufan Wu, Yuan Wang, Chunyan Shen, Cuiping Jiang, Li Liu, Hongxia Zhu \* and Qiang Liu \***

> School of Traditional Chinese Medicine, Southern Medical University, Guangzhou 510515, China; wangzhuxian88@smu.edu.cn (Z.W.); xyq1997@smu.edu.cn (Y.X.); zmnf1988@smu.edu.cn (Z.Z.); huyi12110357@smu.edu.cn (Y.H.); 22020286@smu.edu.cn (Q.Z.); 3161008010@smu.edu.cn (Y.W.); 521wl@smu.edu.cn (Y.W.); shenchunyan@smu.edu.cn (C.S.); jxiaqing126@smu.edu.cn (C.J.); 3188010173@i.smu.edu.cn (L.L.)

**\*** Correspondence: zhuhon@smu.edu.cn (H.Z.); liuqiang@smu.edu.cn (Q.L.);

Tel.: + 86-20-6278-9408 (H.Z.); + 86-20-6164-8264 (Q.L.)

† These authors contributed equally to this work.

**Abstract:** This study aimed to systematically compare licochalcone A (LicA) and glabridin (Gla) (whitening agents) release and permeation from Carbomer 940 (CP) hydrogels with different enhancers, and evaluate the relationship between the quantitative enhancement efficacy and structures of the enhancers. An in vitro release study and an in vitro permeation experiment in solution and hydrogels using porcine skin were performed. We found that the Gla–CP hydrogel showed a higher drug release and skin retention amount than LicA–CP due to the higher solubility in medium and better miscibility with the skin of Gla than that of LicA. Enhancers with a higher molecular weight (MW) and lower polarizability showed a higher release enhancement effect (ERrelease) for both LicA and Gla. The Van der Waals forces in the drug–enhancers–CP system were negatively correlated with the drug release percent. Moreover, enhancers with a higher log P and polarizability displayed a higher retention enhancement effect in solution (ERsolution retention) for LicA and Gla. Enhancers decreased the whole intermolecular forces indrug–enhancers-skin system, which had a linear inhibitory effect on the drug retention. Moreover, C=O of ceramide acted asthe enhancement site for drug permeation. Consequently, Transcutol® P (TP) and propylene glycol (PG), seven enhancers showed a higher retention enhancement effect in hydrogel (ERhydrogel retention) for LicA and Gla. Taken together, the conclusions provide a strategy for reasonable utilization of enhancers and formulation optimization in topical hydrogel whitening.

**Keywords:** carbomer hydrogel; whitening agents; enhancers; enhancement site and mechanism; drug release and permeation

### **1. Introduction**

Hydrogels are used extensively in topical and transdermal drug delivery systems, among which carbomer polymers account for a significant proportion. Carbomer 940 (CP) is used in cosmetic formulation benefitting due to its moderate viscosity and good stability [1]. Drug permeation from hydrogel consists of two processes: the first is drug release from the carbomer matrix and the second is skin permeation. Previous literature has described drug–polymers interaction (mainly H–H bond interaction and Van der Waals forces) [2,3], in which the rheological properties, including the viscosity, storage modulus (G0 ), loss modulus (G00), and phase shift angle (δ) [4], hydration [5], and mesh size [6] of the hydrogels influence drug release. However, the most critical aspects hindering drug permeation are caused by the highly compact structure of the *Stratum corneum* (SC), which limits the

**Citation:** Wang, Z.; Xue, Y.; Zhu, Z.; Hu, Y.; Zeng, Q.; Wu, Y.; Wang, Y.; Shen, C.; Jiang, C.; Liu, L.; et al. Quantitative Structure-Activity Relationship of Enhancers of Licochalcone A and Glabridin Release and Permeation Enhancement from Carbomer Hydrogel. *Pharmaceutics* **2022**, *14*, 262. https://doi.org/10.3390/ pharmaceutics14020262

Academic Editors: Giovanna Rassu and Thierry Vandamme

Received: 24 December 2021 Accepted: 19 January 2022 Published: 22 January 2022

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**Copyright:** © 2022 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/).

efficient delivery of active pharmaceutical ingredients [7,8]. Therefore, a wide range of permeation enhancers are utilized to improve drug release or penetration from the hydrogel system.

Permeation enhancers can enhance drug release from complicated hydrogel systems by reducing the molecular mobility of the systems [9], occupying drug–polymer binding sites [3], etc. Song concluded that permeation enhancers that form hydrogen bonds, such as Span 80 (SP), weaken drug–adhesive interaction, facilitating the release of drug from adhesive [10]. On the other hand, permeation enhancers disrupt the skin lipid arrangement [11,12], change the keratin structure [13], enhance the miscibility of the skin [14], and increase drug partitioning into deeper skin layers [15] to improve the skin permeation of drugs. Plurol® Oleique CC 497 (POCC) showed a preference for occupying sites where skin lipids had interacted with drugs with low PSA and polarizability due to the improved skin–POCC miscibility and stronger interactions [16]. Thus, the physicochemical properties, such as the molecular weight (MW), log P, polar surface area, and polarizability of the enhancers, determine differences in the enhancement effect. Yang systematically evaluated the enhancement action efficacy and sites of different enhancers on drug release and permeation from a patch. It was found that hydrophilic enhancers including Transcutol® P (TP) and propylene glycol (PG) had a better miscibility with matrix carboxyl PSA, indicating their ability to facilitate a higher drug release. In contrast, hydrophobic enhancers including POCC and SP linked with SC lipids more easily, disrupting the lipid arrangement, and thereby improving drug permeation [17]. As a result, demonstrating the molecular interaction of drug–enhancers–CP and drug–enhancers–skin systems is significant, thus shedding new light on the reasonable utilization of enhancers in pharmaceutical and cosmetic preparations.

Licochalcone A (LicA) [18] and glabridin (Gla) [19], flavonoid compounds extracted from the roots of *Glycyrrhiza glabra* L., both have significant anti-melanogenic effects on cellular and animal levels as we previously revealed. However, the poor water solubility and higher log P hinders their transdermal permeation when applied alone, which further influences their practical use. Moreover, it was more difficult for parent LicA to permeate the skin than Gla due to its poorer water solubility and higher log P. Thus, the addition of enhancers represents an effective way to overcome these drawbacks and improve the drug absorption of the two whitening agents. It is expected that higher amounts of Gla and LicA accumulate in the epidermis and dermis rather than in the systemic circulation as melanocytes are located in the basal epidermal layers. The enhancers should firstly accomplish maximum release of the whitening agent from the matrix polymer. Consequently, the ideal enhancers must contribute to the two processes simultaneously. However, to our knowledge, few studies have focused on whitening agents' release and skin delivery behaviors from CP hydrogel based on drug–enhancers–CP (skin) interactions, which has resulted in blindness and uncertainty regarding the utilization of enhancers in whitening formula optimization. Meanwhile, no investigations have systematically compared the enhancing effect of Gla and LicA by different enhancers.

Therefore, for the first time, this study systematically reported the quantitative enhancement efficacy and site of action explaining the drug release and skin absorption of LicA and Gla (whitening agents) from CP hydrogel by different enhancers (Figure 1). Seven enhancers, including POCC, TP, PG, SP, Capryol™ 90 (CP 90), N-methylprolinodone (NMP),and isopropyl myristate (IPM) with different physicochemical parameters (Table 1), were selected. Firstly, Gla–CP and LicA–CP hydrogels with or without enhancers were prepared. The drug release behavior from different CP hydrogels was evaluated by an in vitro drug release experiment. The drug release enhancement effect (ERrelease) and interaction parameters of Gla (LicA)–CP and Gla (LicA)–enhancers–CP hydrogels were demonstrated next. Then, the porcine skin was used to evaluate the enhanced retention and permeation effect of Gla and LicA in solution (ERsolution retention, ERpermeation) and hydrogel (ERhydrogel retention, ERcom) by enhancers, followed by the enhancement site and mechanisms involved in it. In addition, the correlation between the drug release amount,

drug permeation amount, drug retention amount and physicochemical parameters of the enhancers (Table 1), energy of mixing (Emix), and cohesive energy density (CED) were investigated, respectively. These results provide insight into the drug–enhancers–CP and drug–enhancers–skin interactions and the structural characteristics of enhancers, which lays a solid basis for the drug-specific molecular mechanisms of enhancers and pharmaceutical hydrogel design. Moreover, it predicted information for the topical application of enhancers with specific structures for high drug release and skin retention of whitening formulation. cochemical parameters of the enhancers (Table 1), energy of mixing (Emix), and cohesive energy density (CED) were investigated, respectively. These results provide insight into the drug–enhancers–CP and drug–enhancers–skin interactions and the structural characteristics of enhancers, which lays a solid basis for the drug-specific molecular mechanisms of enhancers and pharmaceutical hydrogel design. Moreover, it predicted information for the topical application of enhancers with specific structures for high drug release and skin retention of whitening formulation.

(ERsolution retention,ERpermeation) and hydrogel (ERhydrogel retention, ERcom,) by enhancers, followed by the enhancement site and mechanisms involved in it. In addition, the correlation between the drug release amount, drug permeation amount, drug retention amount and physi-

*Pharmaceutics* **2022**, *14*, x FOR PEER REVIEW 3 of 24

**Figure 1.** Schematic showing Gla and LicA release and permeation from CP hydrogel with enhancers. **Figure 1.** Schematic showing Gla and LicA release and permeation from CP hydrogel with enhancers.


**Table 1.** The physicochemical parameters of different drugs and enhancers. **Table 1.** The physicochemical parameters of different drugs and enhancers.

#### TP 134.20 -1.08 - - - 13.8 39.0 **2. Materials and Methods**

#### *2.1. Material*

**2. Materials and Methods**  *2.1. Material*  Licochalcone A (LicA, purity >98%) and glabridin (Gla, purity >98%) were obtained from Nanjing Spring & Autumn Biological Engineering Co., Ltd. (Nanjing, China). Poly(acrylicacid) (commercial names: Carbomer 940 (CP)), isopropyl myristate (IPM, purity: 98%), diethylene glycol monoethyl ether (commercial names: Transcutol® P Licochalcone A (LicA, purity >98%) and glabridin (Gla, purity >98%) were obtained from Nanjing Spring & Autumn Biological Engineering Co., Ltd. (Nanjing, China). Poly(acrylic acid) (commercial names: Carbomer 940 (CP)), isopropyl myristate (IPM, purity: 98%), diethylene glycol monoethyl ether (commercial names: Transcutol® P (TP), purity: 99%), and 1-methyl-2-pyrrolidinone (NMP, purity >99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Propylene glycol monocaprylate (commercial names: CapryolTM 90 (CP 90)) and polyglyceryl-3dioleate (commercial names: Plurol® Oleique CC 497 (POCC)) were supplied by Gattefossé (Lyon, France). Polyoxyethylenesorbitan monooleate (commercial names: Span 80 (SP)) and propylene glycol (PG, purity > 99%)

were purchased from Damao Chemical Reagent Factory (Tianjin, China). Polyethylene glycol 400 (PEG 400) and cellophane membranes were purchased from Beijing Solarbio Technology Co., Ltd., Beijing, China. All other reagents were analytical grade.

#### *2.2. Preparation of Hydrogels*

The LicA–CP and Gla–CP hydrogels (cargo loading: 5%, *w/w*) were prepared as follows: First, 2 g of CP were dispersed in 100 mL of deionized water and stored at room temperature for 24 h to fully swell the hydrogels. Subsequently, LicA and Gla dissolved in ethanol were added to the CP hydrogels, and the mixture was stirred until it was homogeneous. The pH was to 5 with NaOH solution. Hydrogels with enhancers (drug– enhancers–CP) (cargo loading: 10%, *w/w*) were also fabricated using the same methods. Hydrogels were stored at 4 ◦C and hydrogel films were prepared using the freeze-dried technique.
