1. Introduction
Head and neck cancer (HNC) represents cancers occurred in the head and neck region, which includes the lip and oral cavity, nasal cavity, larynx and pharynx [
1]. Among more than 90% of HNCs are head and neck squamous cell carcinomas (HNSCCs), which accounts for more than 300,000 deaths and 500,000 new cases worldwide annually [
2]. In the United States, HNSCC has been diagnosed as one of the top 10 leading cancers in men in 2022 [
2,
3,
4]. The low survival rate of HNSCC had been proposed to be associated with cancer recurrence, distant metastases, progression of second primary cancers and resistance to chemo/radiotherapy [
5,
6]. Making it as a public health issue that compromise patients’ quality of life.
Tobacco smoking, excessive alcohol, betel quid chewing and high-risk human papillomavirus (HPV) had been documented as risk factors for HNSCC [
7,
8,
9,
10]. Clinical intervention of HNSCC often took place at an advanced stages of the disease due to late diagnosis and poor prognosis, especially among individuals with a lower socioeconomical background [
11,
12]. The most common clinical intervention for HNSCC are surgery, radiotherapy, chemotherapy, or combined therapy which causes numerous side effects while providing some therapeutic effects during the treatment of this disease [
11]. However, even with a successful clinical intervention, approximately 30% of patients treated at an advanced stages of the disease would develop recurrent locoregional or second primary cancers, with the onset of chemo-resistance or radio-resistance and treatment failure as the most prominent underlying factor [
13,
14,
15].
Cetuximab is an epidermal growth factor receptor (EGFR) targeting monoclonal antibody, which is also the first molecular-targeted drug and received approval by U.S. Food and Drug Administration (FDA) as chemotherapeutic agent for HNSCC in 2006 [
5,
16]. A retrospective study in Japan had reported a total effective rate of 57.1%, median progression-free survival (PFS) of 5.5 months and overall survival (OS) of 8.0 months with cetuximab in locally advanced HNSCC, while a total effective rate of 60.0%, PFS of 3.8 months and OS of 5.8 months in distantly metastatic HNSCC [
17]. However, chemoresistance ability of certain mutated cancer cell type such as EGFRvIII showed resistance towards cetuximab, making the drug non-effective as a therapeutic agent towards HNSCC [
18]. In the recent years, pembrolizumab and nivolumab, which both act as anti-programmed cell death receptor 1 (PD-1) immunotherapy drugs had been approved by FDA for recurrent or metastatic HNSCC treatment [
19,
20]. However, most patient previously exposed to the anti-PD-1 monoclonal antibody would develop acquired resistance to immunotherapeutic drug, making it difficult to treat recurrent or metastatic cancers [
21]. Therefore, there is an urgent need for alternative therapeutic agents to overcome the acquired resistance of HNSCC to standard of care. Meanwhile, as therapeutic approaches such as surgical intervention, radiotherapy and chemotherapy are only effective against a limited subgroup of HNSCC patients, and often resulted in additional morbidities, there is a great need for prevention for high-risk potentially malignant lesions and agents that can work effectively as chemoprevention agents or to enhance the effectiveness of chemotherapeutic agents when combined to kill cancer cell.
Natural products are compounds naturally found in natural resources such as plants which possesses biological activities [
22]. In the recent years, natural products had been widely reported for their chemotherapeutic and chemoprevention properties against HNSCC, due to their low cytotoxicity, efficacy against cancers, availability and low cost [
12,
22]. According to National Cancer Institute, U.S., chemotherapy is defined as the treatment that uses drugs to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing, while chemoprevention is defined as the use of certain drugs or other substances to help lower a person’s risk of developing cancer or keep it from coming back [
23]. The beneficial properties of natural products are consistent and could overcome the challenges with current treatment such as acquired chemoresistance, cytotoxicity against normal cells and expensive therapy. Numerous studies had been carried out with various natural products on preclinical models of HNSCC including various HNSCC cell lines and xenograft or carcinogen induced-tumor animal model (
Figure 1). For example, psorachromene, a flavonoid found in Psoralea corylifolia, which had been used in traditional Chinese medicine (TCM) and Ayurvedic had shown therapeutic affects against HNSCC via regulation of EGFR signaling pathways, and other carcinogenesis-related signaling pathway, making it a strong candidate to act as a chemotherapeutic agent [
24]. Other natural products such as calcitrol, had been reported to possess chemoprevention properties against carcinogen induced HNSCC in animal models [
25]. Therefore, natural products could potentially act as adjuvant or neoadjuvant chemotherapy, and chemoprevention of HNSCC.
Activation of complex signaling pathways such as phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) and mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) are important for tumor development, cell survival and angiogenesis, while epithelial to mesenchymal transition (EMT) signaling leads to tumor invasion and migration [
26,
27]. Thus, inhibition of the signaling pathway could lead to cell death, inhibiting the tumor development, and inhibiting metastasis [
27]. Notwithstanding the extensive knowledge at the molecular level of tumor-associated signaling pathways, the chemotherapeutic agents targeting the oncogenic pathways are limited [
28]. To date, Crooker et al. and Rahman et al. have described the use of natural products as chemoprevention agents against HNSCC [
29,
30]. Nevertheless, several natural products such as vitamin A, green tea extract and curcumin which had shown promising result in preclinical studies were reported to show toxicity and limited bioavailability in clinical studies [
31,
32,
33,
34]. Vitamin A has been shown by Shin et al. [
31] and Papadimitrakopoulou et al. [
33] with clinical trials to induce toxicity, majority in terms of oral mucosa and lip inflammation, conjunctivitis, skin reactions, fatigue, joint pain and muscle pain. On the other hand, poor oral absorption of green tea extract and curcumin were observed which limits the bioavailability of both natural products [
33,
34]. Other emerging chemotherapeutic phytochemicals or herbal derivatives against HNC has recently been described by Aggarwal et al. [
12]. However, several newly emerged phytochemicals against or preventing HNSCC, such as actein, calcitriol and psorachromene were not fully addressed. Therefore, in this review, the comprehensive mechanisms of various natural products showing significant preclinical results in HNSCC models in the past 5 years will be discussed.
3. Chemoprevention Properties of Natural Products Against HNSCC Oral Carcinogenesis Mechanism
Oral carcinogenesis often involved formation of abnormalities in the oral tissue known as oral potentially malignant disorder (OPMD) before proceeding to oral squamous cell carcinoma (OSCC), a major type of HNSCC [
109,
110]. Previous analysis had proposed an average malignant transformation (MT) rate of OPMD at 7.9%, while high risk OPMD such as erythroplakia had shown average MT rate of 33.1% [
111]. However, to date, no preventive strategies including the use of drugs and/or surgical procedures is considered the standard of care for OPMDs, thus indicating the need to investigate the chemoprevention properties of various natural products against the oral carcinogenesis mechanism [
112]. Several drugs such as celecoxib, erlotinib and metformin have been investigated for their HNSCC prevention properties via clinical trials on oral premalignant lesions [
113,
114,
115]. However, the use of erlotinib and celecoxib has shown no significant result in reducing oral cancer-free survival rate while possessing higher toxicity [
113,
114], and the use of metformin has shown low clinical response rate (17%) in terms of reduction in lesion size [
115].
In vivo studies involving the use of genetically altered rodents or rodents treated with chemical carcinogens could lead to site specific carcinogenesis, mimicking carcinogenesis in humans [
116]. 4-nitroquinoline 1-oxide (4NQO) acts as a tobacco-mimicking carcinogen which had been widely used in carcinogen-induced HNSCC animal model, mainly due to the similarity in terms of genetic alteration and expression between 4NQO-induced mouse model and human oral carcinogenesis [
117,
118]. Other carcinogens including 7,12-dimethylbenz(a)anthracene (DMBA) and dibenzo[a,l]pyrene (DBP) had also been widely used as HNSCC inducing agent in animal models [
64,
119].
To date, several review studies had introduced various natural products as chemoprevention agents against HNSCC, where vitamin A, green tea extracts and curcumin had shown promising results in preclinical and clinical trials [
29,
30]. However, vitamin A has shown toxicity [
31,
32] while green tea extracts and curcumin [
33,
34] have both shown limitation in bioavailability. In a randomized chemoprevention trial reported by Papadimitrakopoulou et al. [
32], low-dose 13-cis retinoic acid (a derivative of vitamin A) has induced grade 1 (45%), 2 (37%), 3 (15%) and 4 (1%) toxicity, majority in terms of cheilitis, conjunctivitis and skin reactions. Also, treatment of 2 out of 83 patients were halted due to toxicity [
32]. Similarly, a phase II chemoprevention trial by Shin et al. [
31] with combinations of interferon-alpha, 13-cis retinoic acid and alpha-tocopherol had induced mild to moderate non-hematologic toxicity. One patient was reported with severe throat infection due to beta-hemolytic streptococci which required emergency tracheostomy. However, the patient was able to complete the planned treatment after fully recovered from the infection [
31]. Although severe toxicity or side effects due to 13-cis retinoic acid is rare, but it should not be neglected. Green tea extract on the other hand was reported to induce adverse effects such as insomnia, nausea, nervousness and headache, which most likely due to presence of caffeine in green tea extract [
33]. The poor oral absorption of epigallocatechin-3-gallate, most abundant polyphenol in green tea extract, was reported by Tsao et al. [
33], leading to variability in plasma epigallocatechin-3-gallate concentration. Similarly, phase I chemoprevention trial by Cheng et al. [
34] with curcumin has also indicated the poor gastrointestinal absorption of curcumin as the peak serum curcumin concentration was recorded at 1.77µM with 8000 mg daily dosage. The poor bioavailability of both green tea extract and curcumin have introduced difficulties in dosage estimation as absorption varies among patients, which may lead to ineffective treatment. Therefore, in this review, we seek to provide more variety of promising natural products for their chemoprevention properties using 4NQO, DMBA or DBP induced carcinogenesis with animal models [
25,
64,
119].
Table 3 summarized preclinical studies with natural products involved in chemoprevention of HNSCC investigated in the past 5 years.
Preclinical
in vivo studies involving neferine and nimbolide had indicated inhibition in carcinogenesis and reduction in tumor growth of DMBA-induced HNSCC oral carcinogenesis, making both possible chemoprevention agents [
41]. Similarly, calcitriol had successfully inhibited 4NQO-induced HNSCC oral carcinogenesis [
25]. Interestingly, study by Vincent-Chong et al. [
25] with calcitriol on 4NQO-induced carcinogenesis had shown the influence of stage of treatment intervention and duration of exposure to treatment to carcinogenesis, making it prominent to understand the pathway involved in during carcinogenesis and targeting the pathways with respective treatments for an effective intervention.
Calcitriol, nimbolide, and neferine are the three natural products being investigated in the preclinical studies for chemoprevention of carcinogen induced HNSCC oral carcinogenesis, which had also shown promising result by reducing or inhibiting induced-carcinogenesis, by inhibiting carcinogenesis-related molecular mechanisms [
25,
47,
64].
4. Limitation and Future Direction
As multiple signaling pathways and cross-talk between pathways took place during carcinogenesis, and most recurrent or metastatic HNSCC had failed the primary standard of care, an effective treatment for cancer may require combined therapeutic approach such as the use of multiple signaling inhibitors combined with DNA damaging drugs for the most efficient outcome [
26]. Therefore, the synergic or antagonistic effects of natural products with the standard of care such as chemotherapy (cisplatin, cetuximab, pembrolizumab and nivolumab) and radiotherapy should be analysed using preclinical models. However, less than 10% of studies reviewed had reported the combination effects of natural products to the standard of care. For instance, xanthohumol, psorachromene, honokiol, calcitriol and salicylate had been shown to provide synergistic effects with standard of care treatment such as chemotherapy and radiotherapy using HNSCC preclinical models [
24,
25,
42,
44,
45].
As mentioned, the major reason for low survival rate of HNSCC was due to late diagnosis and risk factors associated with HNSCC progression, which would then lead to risk of recurrent or metastatic SCC [
5,
12]. Chemoprevention therapy could potentially act as an important barrier to lower the risk of recurrent or metastatic SCC, and malignant transformation of OPMD, and therefore should be widely studied in the future. Previous prevention of HNSCC involving single agent chemotherapy such as retinoids and isotretinoin possesses high toxicity and low efficacy, indicating the need for development of new chemoprevention agents either as alternative or adjunctive agents for HNSCC prevention. In the current review, only 5 out of 37 studies had explored the potential usage of natural products as chemoprevention therapy using 4NQO/DMBA/DBP-induced oral carcinogenesis [
25,
47,
64,
119]. Furthermore, the stage and duration of intervention of natural products on oral carcinogenesis should also be extensively explored as it has been proposed by Vincent-Chong et al. [
25] to strongly associate with the progression of carcinogenesis. All 6 natural products (
Table 3) are strongly encouraged to proceed with clinical trials for high-risk OPMD patients as previous trials with celecoxib, erlotinib and metformin with mild to advanced OPMD had shown no significant clinical improvements [
113,
114,
115].
Study reported by Dai et al. [
120] using various cancer (melanoma, breast, colon and liver cancer cell lines (H1299, BT549, MDA-MB-231, MDA-MB-468, SW620, MHCC97H and B16F10) in a preclinical study had indicated the crucial role of immunomodulatory roles of natural products in anti-cancer treatments, with the involvement of CD3+ CD8+ T lymphocytes in a co-culture system together with cancer cell lines. Similarly, Cattanaeo et al. [
121] and Neal et al. [
122] have proposed the use of co-culture organoid-tumor reactive T lymphocyte system to investigate the role of anti-PD-1 drugs on T lymphocytes activity, which has been employed by other studies to screen natural product derived drugs and epigenetic inhibitors for the non-cytotoxic T lymphocytes immunomodulating effects [
120,
123]. Apart from
in vitro co-culture system proposed, Verma et al. [
124] had implemented
in vivo RPMOC1 synergic HNSCC animal model to investigate the effects of non-oncological drug, calcitriol, on immunomodulatory of T lymphocytes, which is also the only preclinical study involving synergic HNSCC
in vivo model present at the time of preparing this review. To address this paucity, studies using
in vitro (immune-tumor co-culture system) and
in vivo synergic HNSCC models are urgently needed to investigate the role of natural products listed in
Table 1, 2 and 3 to enhance the anti-tumor effect of immune checkpoint inhibitors.
Among the reviewed natural products, actein, salicylate, tanshinone IIA, xanthohumol, honokiol, trichodermin, psorachromene and protocatechuic acid had been investigated as chemotherapeutic agents with HNSCC cell lines and xenografted animal models, and shown promising targeted molecular mechanisms against HNSCC, making them an ideal candidate for further clinical trials for safety and efficacy analysis [
24,
41,
42,
43,
44,
45,
56,
65]. Furthermore, salicylate and xanthohumol had been investigated to provide synergistic effect with cisplatin and radiotherapy respectively, making them the strongest candidate for future clinical trials for HNSCC or OPMD patients [
42,
44]. Finally, calcitriol, nimbolide and neferine are the major natural products being investigated from this review, which had shown promising chemoprevention properties against induced-carcinogenesis animal models, and are encouraged to further investigate the safety and efficacy with human trials, against high risk OPMD such as erythroplakia and leukoplakia patients via clinical trials [
25,
47,
64].