Strategies for Identification and Clinical Evaluation of Promising Chemopreventive Agents

October 1, 1996

elloff and colleagues have been key players in the recent development of chemoprevention strategies--as initiators of their own studies and minders of others. The succinct summary of their approach is of particular value to oncologists, both because it provides a great deal of data on the current state of chemoprevention research itself and because it draws some useful distinctions between chemoprevention and chemotherapy.

Chemoprevention: Pharmacology or Biology?

elloff and colleagues have been key players in the recent developmentof chemoprevention strategies--as initiators of their own studiesand minders of others. The succinct summary of their approachis of particular value to oncologists, both because it providesa great deal of data on the current state of chemoprevention researchitself and because it draws some useful distinctions between chemopreventionand chemotherapy.

One role for a commentary on such a useful contribution is toset it in a wider context and to ask some questions that Kelloffand coauthors, by their very commitment to the enterprise, areunable to pose. Such a commentary can ask several related questions:

  • Do we know enough about cancer to undertake chemopreventionin the determined pharmacologic way described?
  • What can we learn from the results of completed chemopreventiontrials?
  • Can we draw any other comparisons between chemotherapy andchemoprevention that may help keep us on track?

Do We Know Enough About Cancer?

Asking whether we know enough about cancer to do chemopreventionis not to subvert the current enterprise, nor even to questionthe enthusiastic pursuit of new and better agents. It is, however,intended to remind us that our models of cancer have been evolvingrapidly, and that there have been many surprises over the last10 years. Among these, we can count the semiorganized and cancer-specificnature of the sequence of somatic genetic changes;[1-3] the roleof DNA repair and microsatellite instability;[4-8] the tumor-suppressorrole of inherited genes like BRCA1 with little evidence of

somatic mutation in sporadic tumors;[9,10] the mimicking of asecond "hit" by hypermethylation;[11] clear evidencethat agents thought to cause mutation actually act by selection;[12]the place of apoptosis in carcinogenesis;[13-15] the possibilityof roles for transposons[16-18] and imprint-ing;[19-21] and theheterogeneity of consequences when a gene mutates at differentloci or in the germ line as opposed to somatically.[22-24]

At the very least, this incomplete catalog suggests that we shouldconsider agents beyond those that suppress proliferation or preventmutagenesis. More importantly, it reminds us that we will discoverwhole new processes relevant to carcinogenesis that may undermineour confidence in existing models. Such undiscovered processesmay even explain the failure of agents that, until recently, wefelt secure enough to test in large-scale trials.

In the face of growing evidence of heterogeneity in the cancerprocess, we should adopt an attitude of appropriate humility whilelooking for opportunities both to interrupt the sequence and torevise our models of it. While cultivating such an attitude, itprobably makes sense to eschew the concepts of "toxicity"and "side effects," as these are derived solely fromthe pharmacologic notion that we know what an agent ought to do,and not from a biologic understanding of what it actually does.There are no "main effects" and "side effects,"only effects.

What Have We Learned From Recent Chemoprevention Trials?

The results of recent chemoprevention studies are so at variancewith expectation that it is already clear that some of our modelsare wrong. Human experiments show that modest doses of b-carotenedo not prevent cancer in average-risk[25] or high-risk individuals.[26,27] But what are we to make of an increase in lung cancer[26,27]and large polyps[28] following b-carotene supplementation in high-riskindividuals? A null answer to the chemoprevention question canmean: good hypothesis, good science, but wrong agent. An increasein risk, however, strongly suggests that the model, not the agent,is wrong.

Can We Draw Parallels Between Chemotherapy and Chemoprevention?

One of the parallels between chemotherapy and chemopreventionthat the article does not draw is the outcome of using a singleagent. Extensive clinical trials, in both adults and children,have proven the efficacy of multiple agents, in combination andsequentially. The rationale is so deeply embedded in the way thatwe think about chemotherapy that it is almost subliminal.

Single agents select for clones of cancer cells capable of surviving.Polypharmacy reduces the likelihood of such clones emerging. Selectionoccurs because specific cells are capable, by a variety of means,of resisting the cytotoxic, apoptosis-inducing, and other actionsof the therapeutic agents. Chemopreventive agents, by inducingdifferentiation or reducing proliferation or mutation, are equallycapable of acting as agents of selection, however. Furthermore,the cells that survive chemopreventive agents may be as capableof clonal expansion as any survivor of a cytotoxic agent.

Indeed, it is this kind of selection that may explain why agentsas apparently benign and beneficial as b-carotene can actuallyincrease the risk of lung cancer in individuals known to havelarge numbers of initiated cells. One plausible mechanism forthis outcome is that bcl-2, an inhibitor of apoptosis, may operatevia an antioxidant pathway.[29] Although, on the face of it, b-carotenemay not be a sufficiently powerful antioxidant to induce comparableinhibition of apoptosis, nonetheless, the presence of high concentrationsof even a weak antioxidant may be sufficient to prevent the normalapoptotic death of abnormal cells. The agent then is not workingto prevent mutation-inducing oxidative damage in the DNA of normalcells, but rather, to inhibit the very mechanism by which theorganism sheds abnormal cells.

This is a testable hypothesis. If it is true, what it argues formost strongly is the use of chemopreventive polypharmacy (or diet,of course [30]). If we begin with a single agent and a high-riskgroup, we may indict an agent or class of agents as being uselessor even cancer-promoting when what we have actually discoveredis that our models of cancer are not very good. Some principlesthat derive from both evolutionary biology and chemotherapy applyalso to chemoprevention.

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