Systemic chemoprevention refers to the use of agents that are given orally or intravenously (22). The goal is to reduce the risk of oral cancer by targeting molecular pathways involved in carcinogenesis, including those regulating cell proliferation, cell cycle, apoptosis, angiogenesis, and inflammation (26).

The main studies in this subject are available at Table 1. Among the systemic chemopreventive agents used are vitamin A (27), lycopene (28), celecoxib (29), green tea extract (30), zengshengping (31), bowman-birk (32), beta-carotene (33), curcumin (34), erlotinib (35), and metformin (36). All of these studies (100%) were randomized controlled trials (RCTs). The sample sizes ranged from 23 to 223 participants. The primary clinical response, which aimed to reduce or eliminate the lesions, was the most frequently assessed outcome, reported in 90% of the trials, although only four studies observed a statistically significant benefit for this outcome (27, 28, 31, 34). Histological analysis was the second most reported outcome, mentioned in 60% of the studies, however most of the studies did not show improvement in this outcome. In addition, 40% evaluated protein or molecular biomarkers. Adverse effects were reported in 60% of the studies. The follow-up time varied from 12 weeks to 5 years.

Considering clinical and histological outcomes, only one systemic chemopreventive agent study demonstrated superiority over placebo. Singh et al. (28) conducted an RCT investigating the effects of lycopene on OL, with participants receiving either 8 mg or 4 mg capsules for 3 months (28). A dose-dependent clinical and histological improvement was detected. Lycopene appears to be a promising antioxidant for treating OL by protecting cells against damage and preventing the progression of dysplasia. However, a short two-month follow-up was reported, making it unclear if a long-term effect can be achieved.

Taking into consideration only the clinical outcome, some chemopreventive agents have shown promise as they were superior to placebo: vitamin A resulted in a complete remission rate of 57.1% of OL without side effects (27); ZengShengPing (ZSP) extract showed a positive clinical response in 67.8% of patients, with lesion size decreasing by over 50% after three months of treatment (31); and curcumin improved clinical outcomes in 67.5% of lesions in a RCT, however, side effects such as anemia, skin/subcutaneous tissue disorders, and hypertension were reported (34).

The combination of beta-carotene and vitamin C did not show significant clinical superiority when compared to placebo. Notably, this study also assessed the risk of MT of OL over a 5-year follow-up period (33). The Bowman Birk Inhibitor concentrate (BBIC), also did not result in any significant difference between the placebo and treatment arms regarding clinical response and histological grade or the levels of neu protein, which were assessed as a treatment response biomarker (32).

In recent years, there has been a growing interest in using biomarkers as a tool to improve the efficacy of response detection. These markers can provide valuable information about the biological processes underlying the development of cancer, allowing researchers to identify higher-risk individuals and monitor the response to chemopreventive interventions (37).

First, protein biomarkers were included as secondary outcomes, to investigate both the treatment's efficacy as well as their potential to predict the evolution of the OL. In a randomized phase II pilot study, Papadimitrakopoulou et al. (29) demonstrated that doses of COX-2 inhibitor (celecoxib 100 and 200 mg twice daily) were ineffective regarding clinical and histological outcomes. However, COX-2 expression was found to correlate with OL progression (29). Tsao et al. (30) investigated the expression of VEGF, p53, p16, Ki67, and CD1 after administering green tea extract (GTE). The clinical and histological response rate was higher after GTE in a dose-response effect, but without statistical significance and with insomnia/nervousness reported as side effects (30).

More recently, molecular biomarkers were also incorporated. William et al. (35) in a RCT testing erlotinib, an EGFR tyrosine kinase inhibitor, validated loss of heterozygosity (LOH) as a risk marker for OL MT and its association with an increased number of EGFR (epidermal growth factor receptor) copies. However, erlotinib did not improve cancer-free survival in high-risk patients with LOH-positive OL or with a high number of EGFR copies (35). Gutkind et al. (36) conducted a phase IIa clinical trial in individuals with OL and oral erythroplakia to evaluate metformin's potential to target PI3K/mTOR signaling for MT prevention. The histological response, as well as saliva and blood sample collection, protein expression, and exome sequencing were assessed for outcome analysis. After 12 weeks of metformin treatment, 17% of clinical and 60% of histologic response were observed. Pathogenic mutations, compatible with those found in HPV negative HNSCC, were detected. However, none of the mutated genes were significantly associated with the metformin treatment response (36).

Site-directed chemoprevention is an alternative for managing OL since it is a low-risk and non-invasive strategy. Topical therapies have advantages, such as reducing the toxicity of chemical agents, and can be applied beyond the clinical margins of the lesions to decrease or even eliminate the risk of relapses (38).

Among the topical chemopreventive agents used are bleomycin (39, 40), isotretinoin (41–43), ONYX-015 (adenovirus) (13), ketorolac (cyclooxygenase inhibitor) (44), and dried black raspberries (45) (Table 2). Of the studies included in this review that used topical chemopreventive agents for OL, only 38% were RCTs. The sample sizes ranged from 10 to 57 participants. The primary clinical response was the most frequently assessed outcome (88%), although only two studies observed statistical significant benefit for this outcome (40, 45), followed by histological responses (63%), where only one study showed statistical difference (45). Additionally, 50% of the studies included the evaluation of protein or molecular biomarkers. Adverse effects were reported in 62% of the studies. The follow-up time varied from 3 months to 10 years.

Two studies evaluated the effect of topical bleomycin on OL using different concentrations for 2 weeks. Wong et al. (39) observed that only two out of five cases treated with bleomycin 0.5% showed partial clinical response, and there was no histological improvement. When a higher dose of 1% was tested, three out of seven achieved complete clinical and histological regression, indicating a dose-dependent relation (39). In the RCT by Epstein et al. (40), bleomycin 1% was superior to placebo only regarding clinical response, and not for the histological outcome (40).

Two trials evaluated the effect of topical isotretinoin 0.1% applied three times a day for four months (41, 42). One study observed complete clinical regression in 10% of the lesions (41), while the investigation by Tetè et al. (42) observed three complete clinical regressions (42). Both studies detected a statistical increase of apoptotic bodies, but no difference for bcl-2 expression. Scardina et al. (43) observed complete clinical regression in 85% of lesions after treatment with isotretinoin gel 0.18% twice a day for two months. After a follow-up period of 10 years, the study found no cases of MT (43).

Rudin et al. (13) evaluated a mouthwash with ONYX-015, an adenovirus vector targeting p53-mutant cells. Histological resolution was achieved in 37% of patients, correlating with decreased p53 positivity over time. No significant changes regarding cyclin D1 or Ki-67 expression were found, suggesting that these are not predictive of response to ONYX-015 (13). In an RCT conducted by Mulshine et al. (44) the efficacy of chemoprevention of oropharyngeal leukoplakia using a rinse solution containing 0.1% Ketorolac, a cyclooxygenase inhibitor, was evaluated. Ketorolac was administered twice a day for 33 days, but clinical and histological outcomes did not demonstrate efficacy compared to placebo (44). Mallery et al. (45) evaluated a bioadhesive gel containing 10% freeze-dried black raspberry (BRB) for three months. The results showed significant clinical regression of the lesions and reduced histological grades and LOH events without side effects (45). The bioadhesive gel containing BRB appears to be a promising topical chemopreventive strategy.

There is very little research exploring the potential of chemoprevention of oral cancer, and many of the studies available present significant methodological issues. An important highlight is that many of these studies are not RCTs. Consequently, there is a risk of biases in both patient selection and outcome assessment. Furthermore, the sample sizes in numerous studies are insufficient, challenging the establishment of conclusive findings (13, 41, 42).

There are no standards regarding outcome assessment in this field. While most studies have assessed clinical and/or histological resolution, it should be noted that these analyses may not necessarily indicate a significant impact on the carcinogenesis process. When performing histological analysis, epithelial dysplasia severity is usually assessed. However, this assessment is subject to significant intra- and inter-subjectivity, which is still considered a major issue among pathologists (46).To improve the consistency and reliability of diagnoses, it is advisable to incorporate a binary classification system like the one suggested by Kujan et al. (47) in addition to the conventional WHO system (47). It can reduce confusion and enhance uniformity in the interpretation of findings. Recent studies have included molecular analysis as an outcome measure (35, 36, 45), leading to more effective results. Thus, the main mutations linked to oral carcinogenesis can be assessed, and the risk of cancer development can be better predicted. With the next-generation sequencing approach becoming more readily available, its use will unveil the effects of chemopreventive agents at the molecular level. Consequently, it will allow accurate monitoring of alterations in primary oncogenic mutations or epigenetic modifications (48–50).

Nevertheless, regarding leukoplakia chemoprevention, the ideal outcome is cancer-free survival, which requires a long follow-up and a large sample size. In this sense, multicenter studies are desirable as they can gather a higher number of patients. Moreover, multicenter studies are advantageous as they increase sample heterogeneity and enable the analysis of the efficacy of chemopreventive agents in different populations (36).

Currently, there are limited chemopreventive options available for OL and those that have been evaluated frequently result in significant side effects. Systemic agents offer the advantage of a more widespread effect, which is especially relevant considering the existence of a cancerization field in the oral cavity. This means that altered cells can be found beyond clinically visible lesions (51). Nevertheless, it is essential to note that systemic drugs carry a higher risk of toxicity and side effects, including gastrointestinal symptoms, liver toxicity, increased risk of bleeding, among others. As such, it is crucial to carefully monitor individuals receiving chemopreventive agents for any adverse effects and adjust the dosage or discontinue the agent if necessary. These undesirable effects may be more relevant for patients with comorbidities or who are taking other medications regularly. Therefore, the cost-benefit ratio must be individually assessed.

Topical agents, on the other hand, present a lower risk of significant side effects since they act locally. However, delivering a substance to the oral mucosa has additional challenges. Firstly, some lesions may be located in subsites that are difficult to reach. Secondly, due to the salivary flow, the residence time of drugs in conventional formulations is reduced, potentially decreasing their effectiveness. Therefore, the development of mucoadhesive delivery systems is necessary to achieve better results.

When considering a potential chemoprevention agent, it is essential to also consider their long-term safety since prevention may require longer periods of treatment (31, 33, 35). This is particularly important as prolonged exposure may lead to unexpected adverse effects or toxicity. Moreover, it is important to note that resistance to chemopreventive agents can develop over time, making them less effective.

Considering the criteria of response to treatment, follow-up time, and severity of side effects from studies included in this review, it can be suggested that among systemic chemopreventives, lycopene (28) and green tea extract (GTE) (30) appear to be promising. Both 8 mg and 4 mg lycopene capsules caused histological response, without side effects, although the follow-up time was short, only 2 months. GTE also showed a histological improvement, the side effects reported were mild and a long follow-up period of 27.5 months was reported. Among topical chemopreventives, the application of 10% dried black raspberries (BRB) bioadhesive gel (45) seems promising, showing a histological improvement of lesions and no adverse effects, although the follow-up time was only 3 months. However, it is important to note that these chemopreventive agents were evaluated in a limited number of studies, and additional studies with larger samples and longer follow-up times are needed to confirm these findings.

Even after addressing all the issues raised so far, there are still several challenges associated with implementing chemoprevention for OL on a large scale. Firstly, there is poor awareness about the benefits of chemoprevention and the risk factors for OL, which can result in low participation rates. Early detection of OL remains a challenge, hindering effective implementation of chemoprevention strategies. Additionally, chemoprevention can be expensive and may not be covered by insurance, making it unaffordable to many patients. Nevertheless, healthcare systems and their administrators should consider that the treatment for oral cancer would be much more expensive than its chemoprevention, thus justifying the implementation of these strategies. This highlights the importance of raising awareness of the benefits of chemoprevention and making it more accessible to those who need it, ultimately leading to a decrease in the incidence and burden of oral cancer.

To improve the search for an ideal chemopreventive for treating OL, several strategies can be employed. Firstly, there is a need for a better understanding of the molecular mechanisms involved in oral carcinogenesis to identify the most potential targets for chemoprevention. Secondly, it is crucial to focus on developing safe and effective chemopreventive agents with minimal side effects. Thirdly, there is a need for more standardized RCTs to evaluate the efficacy and safety of potential chemopreventive agents. The future studies on this subject should focus on the identification and selection of high-risk individuals, based on their profile, the subsite of the lesion, use of carcinogenic agents, and especially molecular markers such as LOH and mutations in the main pathways of oral carcinogenesis (35, 36, 52). Also, there should be efforts to increase public awareness about the benefits of chemoprevention and risk factors for oral cancer to increase participation rates. This can be achieved through education and awareness campaigns, targeted screening programs, and community outreach programs. A greater collaboration among researchers, clinicians, policymakers, and other stakeholders is also necessary to implement effective chemoprevention programs at the population level. This may involve various measures such as creating national guidelines for oral cancer screening and prevention, integrating chemoprevention into existing cancer prevention and control programs, and promoting public-private partnerships to facilitate research and development of new chemopreventive agents.