Our previous studies implicated UV-induced oxidative stress as a causative oncogenic insult in an animal model of melanoma, and suggested that the antioxidant NAC may be useful as a chemopreventive agent in human melanoma (23
). Here we describe an ex-vivo
system for studying UV-induced oxidative stress in human nevi, and have used this system to test whether NAC orally-administered to patients can protect their nevi against UV-induced oxidative stress. We found in approximately 50% of patients that nevi removed 3 h following a single 1200 mg dose of NAC, compared to matched nevi removed just prior to drug ingestion, were less susceptible to UV-induced GSH depletion. Our results demonstrate the potential utility of NAC in protecting against pro-carcinogenic oxidative stress induced by UV exposure, and further suggest and support a novel paradigm for melanoma chemoprevention, as discussed below.
Given the potent antioxidant capacity of NAC and our ability to optimize its delivery to nevi in vivo
, one might have expected a higher rate of efficacy in our model system. There are several variables, however, which we did not account for that may have affected either UV-induced oxidative stress or NAC-mediated protection. First, we do not have genotyping information our patients for MC1R, which regulates both UV-induced ROS production and metabolism in melanocytes (17
). Although we attempted to control for the degree of UV-induced oxidative stress by incorporating a ‘pre-drug’ nevus from the same patient, it is possible that MC1R polymorphisms in some patients may have resulted in a different spectrum and/or levels of ROS or antioxidant response that may be more refractory to NAC protection. In addition, we did not alter drug dosage based on patient weight, nor account for whether the drug was taken immediately prior to or following eating, which may have affected drug efficacy. It is also important to note that we assumed that optimal conditions for drug delivery to nevi were similarly optimal for restoration of depleted GSH. Although we may not have fully optimized the experimental conditions, the fact that we did observe a protective effect in a number of patients does satisfy a proof-of-principle that NAC may protect nevi against oxidative stress if taken in advance of UV exposure.
According to recent U.S.-based epidemiologic studies, melanoma is more common and increasing at a faster rate in men then women (33
). Although the basis for these trends is unclear, we note that markers of systemic oxidative stress such as γ-glutamyltransferase (34
) are increased in males compared to females (35
). Moreover, animal studies revealed that females appear to be protected against oxidative stress due to endogenous estrogens (36
). Interestingly, we found that 3 of 10 (30%) non-responders and 1 of 9 (11%) responders () were female, suggesting the possibility that female sex may be a predictor of non-responsiveness to NAC in our patients. However, the higher rate of male responders (odds ratio 3.2, 95% confidence interval 0.20-202) was not statistically significant (p=.58). We will be able to pursue this question further in a larger future study (see below).
There are many obstacles associated with a conventional chemoprevention approach for cancer, which usually involves chronic drug administration (). First, it is difficult to maintain and monitor patient compliance for an extended period of time. In addition, there may be unpredictable toxicities associated with chronic ingestion of any agent. Even if informative biomarkers are available and assessed at intermediate time points to monitor drug delivery and action, it is generally not possible to assess clinical benefit until the end of the trial (i.e. did the patients in the experimental/intervention group develop less cancer?). Another potential problematic consideration is that the chemopreventive agent may not be administered in conjunction with the specific oncogenic stimulus, which for many cancers is unknown (). For melanoma, the long latency time and low (annual) risk of tumor development are such that large numbers of patients would need to be treated and monitored for many years to determine whether a given preventive agent is effective. Finally, a combination of these factors may yield unanticipated adverse (or paradoxical) results as observed in various prevention trials of antioxidants (37
). For example, patients who took β-carotene and retinol for two years exhibited an increased risk (relative risk 1.38) of lung cancer (38
), and a French study recently reported that mixed antioxidant supplementation over seven years was associated with increased risk (hazard ratio 1.68) of skin cancer in women (39
Several agents targeting oxidative stress have been considered for melanoma chemoprevention (40
). These include epigallocatechin-3-gallate (EGCG), a polyphenol antioxidant present in green and black teas, which decreases inflammation and induces cell cycle arrest and apoptosis in vitro
). Orally-delivered EGCG has also been shown to inhibit tumor promotion and metastasis in a mouse melanoma model (43
). Though these data for EGCG are encouraging, a prospective cohort study in postmenopausal women found no association between tea drinking and melanoma incidence (44
). Other proposed antioxidants for melanoma chemoprevention include Vitamin E, β-carotene, lycopene, flavonoids, resveratrol and selenium, but as with EGCG, there is lack of compelling data in humans (40
). Moreover, the proposed use of these agents would be subject to all the obstacles associated with conventional chemoprevention regimens noted above.
The antioxidant NAC has not been formally tested as a chemopreventive agent in melanoma. Our data, however, suggest that NAC may be useful – not
in a conventional chemoprevention protocol, but rather as a chemopreventive agent in the context of acute UV exposure
. A chemoprevention strategy in which NAC is taken episodically, rather than chronically, in anticipation of sun exposure, would bypass most of the obstacles associated with conventional chemoprevention. We envision that patients with nevi, who are at increased risk for melanoma (45
), could take NAC as a prophylactic ‘sunburn pill’ in anticipation of sun exposure to protect their nevi against UV-induced oxidative stress (). If successfully implemented, this strategy would have a presumed benefit
, since a reduction in pro-oncogenic oxidative stress in nevi over the course of many UV exposures would be predicted to decrease long-term (lifetime) melanoma risk. In addition, there would be less oxidative damage over time in isolated melanocytes, from which the majority of melanomas arise (2
This study paves the way for a formal trial in which NAC-mediated protection could be assessed in the context of incident (in vivo) UV exposure, which would better represent the setting in which most people acquire UV-induced oxidative stress in their skin and nevi over time. Finally, we note that this novel chemopreventive strategy may be useful for other cancers triggered by known environmental stimuli.