Goals of dissertation research. Lung cancer is currently the leading cause of cancer deaths among men and women and has a survival rate of only 16%. Because nearly 90% of lung cancer cases are attributed to smoking tobacco and tobacco related lung cancer has a latency period of 20–30 years from time of exposure to invasive disease, intervention before invasive disease with a chemical agent, termed chemoprevention, is a well suited approach for this particular disease. Research suggests that many natural compounds, often found in the human diet, have chemopreventive properties and are actively being explored and/or can make excellent lead compounds. Our lab, using rational drug design techniques, developed a novel isoselenocyanate compound known as phenylbutyl isoselenocyanate (ISC-4) based on naturally occurring compounds found in cruciferous vegetables. The central hypothesis of this research was that ISC-4, based on its design, would function as a potent and effective anti-cancer agent and could be used as a chemopreventive agent.

Specific Aims. Specific Aim 1: To determine the effects of replacing the sulfur atom of a panel of phenyl alkyl isothiocyanates with selenium to form a new panel of phenyl alkyl isoselenocyanates (Chapter 2). A structure activity study was performed on the panel of parent isothiocyanate compounds and the resulting panel of isoselenocyanate compounds to determine if mechanistic characteristics were retained and to ascertain a reason for increased activity. Specific Aim 2: Testing the hypothesis that ISC-4 has potential as a chemopreventive agent (Chapter 3). This drug was found to potently target and inhibit carcinogen bioactivation by cytochrome P450 enzymes, including the tobacco specific nitrosamine procarcinogen, N-nitrosyl ketone (NNK). When bioactivated, NNK can lead to mutations in DNA and ultimately result in tumorigenesis. A series of experiments were conducted to assess chemopreventive properties including bioavailability, anti-initiation, and DNA adduct studies, all of which were needed as proof of concept before performing an animal bioassay. Specific Aim 3: Testing ISC-4 as a chemopreventive agent in a lung cancer chemoprevention bioassay (Chapter 4). Using an A/J mouse model, ISC-4 at varying doses and along with its sulfur analog PBITC were tested for the ability to inhibit NNK induced lung tumorigenesis over a 24 week period of time.

Original breakthroughs and findings. Isoselenocyanate (ISC) compounds were actually found to have increased activity with cellular thiols compared to their corresponding sulfur analogs leading to the hypothesis that specific protein modifications and redox activity in the cell are responsible for the increased activities seen with the ISC compounds. As Phase I cytochrome P450s were identified early as a significant protein target of ISC-4, the cancer chemopreventive properties of this compound were explored. ISC-4 was found to be orally bioavailable, able to transcriptionally induce Phase II detoxification enzymes, and inhibit DNA adduct formation in lung which showed that ISC-4 had anti-initiation properties that warranted further investigation as a lung cancer chemopreventive agent. In a large scale lung cancer chemoprevention bioassay using A/J mice, ISC-4 was able to inhibit NNK induced lung tumorigenesis by 86% when fed in the diet at 0.38 µmol per gram. ISC-4 treated mice also had a lower percentage of adenomas compared to hyperplasia in the lung, which suggests ISC-4 may have anti promotion or progression properties as well. However, when compared to its sulfur analog PBITC, ISC-4 is not as well tolerated and therefore may be more suited in anti-progression and chemotherapy, at least for lung cancer.