Former and current smokers and individuals who have been successfully treated for aerodigestive cancer would greatly benefit from drugs that prevent progression of lung neoplasia. This group of people is vast, comprising over one-third of the adult population of the United States. Isotretinoin exhibits excellent potential as a lung cancer chemopreventative in vitro and for this reason has been the subject of a number of clinical trials, with generally disappointing results. We reasoned that the discrepancy between lab and clinic could be explained by pharmacokinetics-orally administered isotretinoin is >99 percent bound by blood protein, leaving too little to diffuse to the target lung epithelium-and we conducted a pilot study with inhaled isotretinoin aerosol in carcinogen-treated A/J mice, with quite promising results. From this and other pilot studies, we found that: retinoic acid receptor (RAR) upregulation in the lungs correlated with efficacy; inhaled isotretinoin did not upregulate liver RARs; and, oral isotretinoin upregulated liver, but not lung, RARs. Based on these observations and in vitro results, we derived a hypothesis for the mechanism of action: upregulated RARs inhibit the growth-stimulating action of AP-1 whose activity in the absence of an inhibitor would be increased via a Ras initiated cascade. The proposed research will test the hypothesis in NNK-treated A/J mice by determining AP- 1 DNA binding activity and upregulated biomarker proteins or mRNAs in both isotretinoin-exposed and unexposed animals. In addition, RAR upregulation as a direct predictor of efficacy will be examined and the possibly complex relationships between inhaled dose and efficacy will be explored. RAR and RXR upregulation and other endpoints will be determined in the lungs of mice exposed to a range of pulmonary doses for up to 11 days. In long-term exposures of up to 34 weeks, tumor multiplicities will be determined in addition to biomarkers. Completion of the proposed research will have a number of benefits including: providing a paradigm for studying inhaled drugs that may act similarly to isotretinoin; validating RAR levels as an indicator of an efficacious dose; and dose guidance for potential studies in humans.
