We performed a Phase 2a dose-finding and efficacy trial of topically administered POH in a cream formulation having previously documented suitable skin tolerability in a Phase 1 study (9
). Trial participants had sun-damaged skin with at least two clinically diagnosed AK on each dorsal forearm to document at least a moderate level of risk for non-melanoma skin cancer. There were multiple primary endpoints (i.e. normalization of quantitative skin histopathologic scores and skin nuclear chromatin pattern) and secondary endpoints (i.e. skin biopsy PCNA, apoptosis, and p53 biomarkers indices).
Our interest in POH as a topical chemopreventive agent for skin cancer was fueled by positive preclinical studies, documenting considerable activity in both UVB-induced murine SCC and melanoma (2
). There is increasing interest in the development of topically administered agents that could slow the progression of moderately or severely sun-damaged skin to non-melanoma skin cancer and the strong preclinical chemopreventive activity and excellent tolerability of topically administered POH make it a strong candidate for further clinical development.
As reported, there was a borderline difference that approached statistical significance in the change in sun-damaged dorsal forearm biopsy histopathologic score between study participants treated with placebo cream versus those treated with 0.3% (low-dose) POH; however, the high dose (0.76%) POH group did not show a similar trend. This was surprising, given the strong preclinical results; and the apparent lack of dose-dependence indicates that either the drug is not effective or the pharmacokinetic profile (i.e., the residence time of the drug at the site of action) was not suitable. However, in contrast to the results obtained using the histopathologic scoring system, dose dependent effects on reduction of nuclear abnormalities were observed using karyometric analysis. This supports the hypothesis that the negative histopathologic results were due to inadequate exposure of the drug to its target, most likely due to sub-optimal pharmacokinetic properties of the formulation.
Using karyometric analysis, we observed that the higher dose of POH (but not the lower dose of POH) was associated with a statistically significant reduction in the proportion of skin biopsy nuclei deviating from normal (i.e. p<0.01 with a power of test of 0.99). In case-by-case comparisons, it was difficult in this study to establish the statistical significance of the high dose POH versus placebo, due to the limited sample sizes of nuclei recorded for each participant. Of those participants treated with topically administered higher dose POH, 31% had a statistically significant reduction in proportions of nuclei deviating from normal, versus just 17% in the placebo group. This trend to less deviation from normal may very well represent a beneficial cancer preventive effect that will continue to be studied in future clinical trials using karyometric analysis as a biomarker endpoint.
Neither p53, PCNA, nor apoptosis were affected by the high or low dose POH concentrations administered topically twice daily over three months. These negative biomarker results match those observed in our prior positive skin cancer chemoprevention studies of oral vitamin A and topical difluoromethylornithine (18
). On the other hand, karyometric analysis of nuclear chromatin abnormality in baseline and end-of-study sun-damaged skin once again has proven useful as an “integrating biomarker” of response to chemopreventive agent activity (13
The only adverse event that occurred frequently was a mild or moderate local reaction (termed rash, redness and/or erythema) that occurred at a rate of 56% and 54% for the low and high dose POH dose, respectively. However, this reaction may very well be due to the vehicle cream since the placebo group reported this same type of event in 43% of participants and this proportion is not statistically significantly different from that observed in the treatment groups. Similarly, the Phase 1 trial also demonstrated a lack of any serious adverse events (9
) and no significant differences in the mild, study-related adverse events between treatment and placebo groups.
Obviously, topical POH cannot be effective as a skin cancer chemoprevention agent if it penetrates poorly into epidermal skin, or (possibly) if it rapidly penetrates the skin transdermally. An ongoing direction in our research program includes both in vivo preclinical and clinical projects to compare the skin penetration of the parent POH topical formulation and a novel prodrug topical formulation, engineered specifically to concentrate POH in the epidermis (24
). Development of true pharmacokinetic profiles (concentration vs. time curves) of percutaneous absorption and transdermal penetration of topically administered drugs in the context of a trial such as this one are impractical due to the excessive numbers of biopsies that would be required. Development of a prodrug of POH engineered with appropriate lipophilicity will allow for ex vivo human skin models to be developed that can compare the skin pharmacokinetics of parent POH and POH prodrugs to ensure adequate residence time of the active form of the drug.
In conclusion, the results of this study suggest that topically administered POH at 0.76% twice daily for 3 months has a modest effect in reducing nuclear chromatin abnormality in moderately to severely sun-damaged skin. Further studies of prodrug formulations of POH may result in a more active formulation of this novel topical agent that could be utilized in future non-melanoma skin cancer chemoprevention studies.