Anti-cancer activity of sustained release capsaicin formulations

Abstract

Capsaicin (trans-8-methyl-N-vanillyl-6-noneamide) is a hydrophobic, lipophilic vanilloid phytochemical abundantly found in chili peppers and pepper extracts. Several convergent studies show that capsaicin displays robust cancer activity, suppressing the growth, angiogenesis and metastasis of several human cancers. Despite its potent cancer-suppressing activity, the clinical applications of capsaicin as a viable anti-cancer drug have remained problematic due to its poor bioavailability and aqueous solubility properties. In addition, the administration of capsaicin is associated with adverse side effects like gastrointestinal cramps, stomach pain, nausea and diarrhea and vomiting. All these hurdles may be circumvented by encapsulation of capsaicin in sustained release drug delivery systems. Most of the capsaicin-based the sustained release drugs have been tested for their pain-relieving activity. Only a few of these formulations have been investigated as anti-cancer agents. The present review describes the physicochemical properties, bioavailability, and anti-cancer activity of capsaicin-sustained release agents. The asset of such continuous release capsaicin formulations is that they display better solubility, stability, bioavailability, and growth-suppressive activity than the free drug. The encapsulation of capsaicin in sustained release carriers minimizes the adverse side effects of capsaicin. In summary, these capsaicin-based sustained release drug delivery systems have the potential to function as novel chemotherapies, unique diagnostic imaging probes and innovative chemosensitization agents in human cancers.

Introduction

Capsaicin is the spicy pungent ingredient of chili peppers. It is a potent analgesic agent and a common ingredient in over-the-counter pain-relieving lotions and creams (Bannerjee & McCormack, 2020; Basith, Cui, Hong, & Choi, 2016; Evangelista, 2015). The analgesic activity of capsaicin is mediated by the transient receptor potential vanilloid (TRPV1) receptor. Capsaicin is a high affinity agonist of the TRPV1 receptor (Andresen, 2019; L. Li et al., 2021). However, several lines of evidence show that the biological functions of capsaicin may be mediated by TRPV1-dependent or TRPV1-independent pathways (Arul & Ramalingam, 2020; S. Zhang, Wang, Huang, Hu, & Xu, 2020).

Early studies showed that capsaicin displayed robust chemopreventive activity in a several types of human cancers including lung, prostate, pancreatic, and skin cancer. Subsequent research demonstrated that capsaicin suppressed the growth and progression of human breast, lung, prostate, gastric, renal, oral cholangiocarcinoma and hepatocellular carcinoma (Arul & Ramalingam, 2020; Basith et al., 2016; Chapa-Oliver & Mejía-Teniente, 2016; Clark & Lee, 2016; Srinivasan, 2016; S. Zhang et al., 2020) in cell culture and animal models. Although, an overwhelming majority of research papers show that capsaicin displays growth-inhibitory effects in human cancer cells (Arul & Ramalingam, 2020; Basith et al., 2016; Chapa-Oliver & Mejía-Teniente, 2016; Clark & Lee, 2016; Srinivasan, 2016; S. Zhang et al., 2020), a few studies have suggested that capsaicin promotes the survival and growth of breast, colon and skin cancers (Bode & Dong, 2011;). Erin, Boyer, Bonneau, Clawson, & Welch, 2004 and Erin, Zhao, Bylander, Chase, and Clawson (2006) showed that the administration of capsaicin at high doses (125 mg capsaicin/kg body weight) increased breast cancer aggressiveness and promoted mammary tumor metastasis to the lung and heart (Erin et al., 2004; Erin et al., 2006). An important point to note here is that the authors used extremely high doses of capsaicin (125 mg/kg bodyweight) for their experiments. The aim of their studies was to demonstrate that capsaicin caused denervation of sensory neurons in breast carcinomas and such denervation promoted breast cancer metastasis (Erin et al., 2004; Erin et al., 2006). In fact, several publications have demonstrated the growth-suppressive activity of have shown that the capsaicin in breast cancer using cell culture and orthotopic mouse models. Similarly studies by Yang et al. (2013) have shown that the capsaicin promoted the metastasis of colon cancers in CT-26 syngenic mouse models (Yang et al., 2013). However, they used an atypical protocol to measure the effect of capsaicin on metastasis of colon tumors in syngenic mice. Conventionally, the protocol to perform such experiments is to first establish the metastatic tumors in the mice (Guerin, Finisguerra, Van den Eynde, Bercovici, & Trautmann, 2020). Subsequently, the mice should be randomized the mice into control and treatment groups. The treatment group should be administered capsaicin (via diet, or oral gavage or osmotic pumps or intraperitoneal injections) for a period of 3–4 weeks. After the treatment period the mice should be euthanized and the effect of capsaicin on the number of metastatic foci should be determined. However, Yang et al. (2013) treated CT-26 cells in vitro with a high dose of capsaicin (100 μM) for 48 h and then injected these cells intravenously (via the tail vein) of BALB/C mice. The fact that they used a different protocol to perform the mice experiments may explain the aberrant results obtained in their studies. This is clearly exemplified by the data of Caetano et al. (2021) who demonstrated that capsaicin did not possess any tumor-promoting activity on colon carcinogenesis (Caetano et al., 2021). In fact, several publications show capsaicin inhibits the growth of colon cancers in cell culture and mouse models (Jin et al., 2014; Lee & Clark, 2016).

A few published reports suggested that the administration of capsaicin in Swiss albino mice induced duodenal adenocarcinomas (Hwang et al., 2010; Toth & Gannett, 1992; Toth, Rogan, & Walker, 1984). However, a close survey of their data show that the appearance ofsuch tumors were not related to the administration of capsaicin. Studies by the research group of Bode et al. showed that the co-administration of capsaicin with 7,12-dimetylbenz(a)anthracene (DMBA) and tetradecanoylphorbol-13-acetate (TPA) increased the incidence of skin tumors in TRPV1 knockout mice. The study did not include a group of mice treated exclusively with capsaicin. Bley, Boorman, Mohammad, McKenzie, and Babbar (2012) have rigorously analyzed the data published in these papers and inferred that the capsaicin could be increasing the number of TPA-induced skin tumors by enhancing the delivery and bioavailability of TPA in the skin of these mice (Bley et al., 2012).

Chanda et al. (2007) explored the direct effect of capsaicin on skin tumorigenesis was in female hemizygous Tg.AC mice (Chanda et al., 2007). They administered varying doses of capsaicin (via topical application) on the dorsal skin of female Tg.AC mice for 26 weeks. Specifically, the doses of capsaicin used in this study ranged from 0.64 mg capsaicin/ mouse/week-2.56 mg capsaicin/mouse /week. The volume of capsaicin solution applied on the skin was 0.1 ml. It must be noted that the concentration of capsaicin applied to the treated skin areas was quite high, as the lowest capsaicin dose level in the present study (0.64 mg/0.1 ml) corresponds to a concentration of 20.9 mM. After 26 weeks, the authors observed no increased dermal masses or preneoplastic and neoplastic lesions in skin (exposed to very high concentrations of capsaicin). Based on their data, they concluded that capsaicin did not display any oncogenic activity on the skin of mice. In summary, all these studies did not provide any concrete evidence that capsaicin promoted the growth of colon, breast and skin cancers.

Apart from its direct growth-suppressive activity, capsaicin sensitizes human cancer cells to the cytocidal effects of standard-of-care chemotherapeutic agents. These include radiation-therapy, synthetic small molecules, conventional chemotherapeutic drugs, as well as targeted signal-transduction inhibitors (Friedman et al., 2019; S. Zhang et al., 2020). It is well established that prolonged treatment with cancer chemotherapy drugs (or radiation) leads to the acquisition of drug-resistance in human cancers (Alfarouk et al., 2015; Vasan, Baselga, & Hyman, 2019). Such resistance is one of the reasons contributing to the dismal survival rates of many cancers. We believe that the ability of capsaicin to sensitize tumors to the effects of chemotherapy drugs may have important clinical applications in cancer therapy. The addition of capsaicin to existing anti-cancer drug regimens may improve the therapeutic index of such combination therapies and improve health outcomes of cancer patients. Since capsaicin is already used in the clinic for its pain-relieving effect, the idea of repurposing this drug for cancer therapies has generated considerable enthusiasm amongst cancer researchers (Efferth & Oesch, 2021; Kale, Amin, & Pandey, 2015). Taken together, such facts emphasize the translational potential of capsaicin-based therapy in patients.

The chemosensitization and anti-tumor activity of capsaicin involves multiple molecular pathways including, inhibition of cell proliferation (Ali Al-Samydai, 2019; Chan, Azlan, Ismail, & Shafie, 2020), induction of apoptosis (Clark & Lee, 2016; S. Zhang et al., 2020), regulation of autophagic pathways (Chang, Islam, Liu, Zhan, & Chueh, 2020; C. H. Choi, Jung, & Oh, 2010; Lin et al., 2017; Ramos-Torres, Bort, Morell, Rodríguez-Henche, & Díaz-Laviada, 2016) and alteration of the pharmacokinetics of conventional chemotherapeutic drugs (Wang, Zhu, Zhang, Zhai, & Lu, 2018) (Fig. 1). In addition, an important mechanism underlying the growth-suppressive activity of capsaicin is its ability to inhibit tumor angiogenesis and metastasis (Chakraborty et al., 2014; Friedman et al., 2019; Min et al., 2004; Pyun et al., 2008). All these facts emphasize the therapeutic potential of capsaicin as a useful anti-cancer drug, both as a single agent or in combination with existing chemotherapeutic drug regimens.

The clinical development of capsaicin as a viable anti-cancer drug is hampered by three factors, 1) the low solubility of capsaicin in aqueous environments, 2) the short biological half-life and bioavailability of capsaicin in vivo, 3) the adverse side effects of oral administration of capsaicin. The solubility of capsaicin in water is extremely low (0.0013 g/100 ml water). Therefore, all cell culture studies exploring the use of capsaicin as an anti-cancer drug have used solutions of capsaicin in water mixed with low concentrations of organic solvents such as DMSO or ethanol (Costanzo, Yost, & Davenport, 2014; Turgut, Zhang Newby, & Cutright, 2004). Secondly, capsaicin has a short biological half-life in plasma and is rapidly eliminated from the (O'Neill et al., 2012; Reyes-Escogido Mde, Gonzalez-Mondragon, & Vazquez-Tzompantzi, 2011; Rollyson et al., 2014) (Fig. 2). Kawada et al., (1984) analyzed the metabolism of capsaicin after intraperitoneal (i.p) injection in anesthetized male Wistar rats. They observed that capsaicin was rapidly metabolized after injection. The maximal amount of capsaicin was detected in the thigh venous blood 16 min after the intraperitoneal injection. Furthermore, the amount of capsaicin in the blood decreased rapidly within 40 min after injection. The half-life of capsaicin was found to be approximately 12 min in the blood (Teruo Kawada & Iwai, 1985; T. Kawada, Suzuki, Takahashi, & Iwai, 1984). The third disadvantage with capsaicin is that it causes skin redness, hyperalgesia, nausea, intense tearing in the eyes, conjunctivitis, blepharospasm (sustained, forced, involuntary closing of the eyelids), vomiting, abdominal pain, stomach cramps, bronchospasm, and burning diarrhea in patients (Drewes et al., 2003; Evangelista, 2015; Hammer, 2006). Clinical trials exploring the pain-relieving activity of capsaicin have shown that such side effects have resulted in patients discontinuing use of the drug.

A strategy to overcome all these hurdles is to entrap capsaicin within polymeric drug carriers to generate sustained release formulations. The encapsulation of capsaicin in sustained release systems ensures that the drug is uniformly dispersed within the polymeric matrix (Chittepu-Reddy, Kalhotra, Revilla, & Velazquez, 2018; Rollyson et al., 2014). The capsaicin is released from the polymer in a slow, steady and prolonged manner which improves its solubility properties. Some forms of sustained release drugs contain chaotropic salts, surfactants and co-surfactants which enhance the ability of capsaicin to disperse uniformly in the aqueous microenvironment (Chittepu-Reddy, Kalhotra, Revilla, & Velazquez, 2018; Rollyson et al., 2014). The capsaicin is homogenously entrapped within the polymeric matrix scaffold and so it is not available to intracellular enzymes for degradation. Drug release studies and biodistribution experiments have shown that sustained release platforms considerably increase the amount of capsaicin being delivered to the blood (and specific organs) in mouse and rat model systems. Finally, the gradual release of capsaicin ensures that only a small amount of the drug is present in the blood which reduces the incidence of gastrointestinal and skin irritation in patients.

Clinical studies reveal that extended release capsaicin formulations are used to combat neuropathic pain in patients. The long acting formulation of capsaicin (the transdermal patch Qutenza) is used in the clinic to relieve neuropathic pain (associated with postherpetic neuralgia) and diabetic nerve pain (Burness & McCormack, 2016; Uceyler & Sommer, 2014). The clinical application of QUETENZA raises the possibility that sustained release capsaicin formulations may have applications in the treatment of other diseases like cancer. A plethora of published papers have provided evidence that slow-release capsaicin formulations display potent anti-tumor activity in cell culture systems and mouse models (S. Zhang et al., 2020). Most importantly, these long-acting capsaicin formulations selectively kill cancer cells and have minimal growth-suppressive activity on normal cells.

An exciting development has been the discovery of sustained release formulations which are capable of releasing capsaicin and chemotherapeutic drugs simultaneously. Polymeric drugs capable of releasing paclitaxel, gefitinib and irinotecan with capsaicin have shown robust anti-cancer activity in multiple in vivo model models of human cancer (Lan et al., 2019; P. Parashar et al., 2019a; L. Wang et al., 2017). A cutting edge application of capsaicin-based sustained release formulations has been as imaging probes for the early detection and diagnosis of cancers. It is well established that one of the challenges in the treatment of lung cancer, hepatocellular carcinoma and pancreatic cancer is the lack of methods for early detection of these tumors in patients (Cassim et al., 2019; Gheorghe et al., 2020; Midthun, 2016; Parikh et al., 2020; X. R. Wang et al., 2017). Taken together, we believe that capsaicin-based sustained release formulations may have multiple clinical applications in the detection and treatment management of cancers. In particular, the chemosensitization ability of slow-release capsaicin drugs and their ability to function as imaging agents (for early diagnosis of cancers) has the potential to improve the health outcomes of cancer patients in the clinic. The primary objective of this manuscript is to describe the pharmacological and biological properties of capsaicin-based sustained release formulations which have been explored for the treatment of human cancers. An asset of these long-acting capsaicin formulations is that they selectively kill cancer cells and have minimal growth-suppressive activity on normal cells.

Several review articles describing the analgesic activity of capsaicin-sustained release formulations can be found in literature (Arora, Campbell, & Chung, 2021; Fattori, Hohmann, Rossaneis, Pinho-Ribeiro, & Verri, 2016). However, none of them have provided a comprehensive overview about the anti-cancer activity of capsaicin-based sustained release drug delivery systems. Our manuscript fills this void of knowledge. We believe that this review article is timely, relevant and will provide novel insights (involving the pharmacology of capsaicin formulations) to a wide spectrum of researchers working in the field of cancer biology.