Response to: “Deceptive argumentation against diagnostic microdosing of anticancer drugs” by Dirk Theile and Gerd Mikus (Letter dated February 13, 2014)

Dear Sir, The purpose of our minireview was achieved by receiving this thoughtful response initiating an international discussion of the appropriateness of diagnostic microdosing of cytotoxic chemotherapeutic agents. We would like to point out that we are not diminishing the general usefulness of microdosing in drug development. What motivated us to write this article is that it is illogical at this time to test microdoses of platinum-based chemotherapies such as carboplatin in newly diagnosed lung cancer patients without fully understanding the potential risks of inducing early adaptive cellular changes associated with drug resistance. Thiele and Mikus took issue with our discussion by titling their correspondence “Deceptive argumentation. ” We disagree that we are being deceptive in any way and apologize if it seems so. First, we would like to address some misconceptions by these authors. The authors appear to have confused the terms cytostatic and cytotoxic. Our manuscript addresses microdosing of nonspecific cytotoxic agents such as cisplatin and carboplatin rather than nontoxic cytostatic agents. Second, the authors cited a reference showing that low doses of ritonavir lead to underproportional exposures due to abolition of autoinhibition. Although this is interesting from a pharmacokinetic point of view, we do not see how this is relevant to our discussion. Using ritonavir as an example is misleading as this drug is known to be one of the most potent CYP3A inhibitors. If anything, this example illustrates how very low doses of chemotherapeutics, as in diagnostic microdosing, could have unpredictable effects at the cellular/molecular level. Finally, the authors pointed out that the use of peripheral blood mononuclear cells (PBMCs) to predict clinical response to chemotherapy is inconclusive by citing a 1996 study in 66 patients. More recent evidence based on a clinical study in 591 patients showed a significant correlation between DNA repair capacity in PBMCs and overall survival. Given this newer study, our recommendation of using this method would be safer than employing diagnostic microdosing of first-line chemotherapeutics in newly diagnosed lung cancer patients. The authors pointed out that concentrations of the chemotherapeutics used in some of the in vitro studies we cited were not “microdose-like” and that the in vitro colony-forming assay has disadvantages. We agree that the drug concentrations used in some of these studies were not accurately reflective of microdosing and that the in vitro sphere-forming, or colony-forming, assay has problems, but these studies were used to illustrate our point that exposures to very low doses of chemotherapeutics can, under certain circumstances, lead to the development of resistance. The authors further pointed out that cisplatin concentrations used in in vitro studies should be scaled based on cisplatin plasma levels seen in patients following a therapeutic dose. We agree in principle that we should test concentrations that are achieved at the actual target following a microdose. The problem is that currently, no one has defined intracellular concentrations of cisplatin following a microdose. It is well-known that plasma levels of cisplatin are not predictive of clinical response or drug delivery to the tumor. Serum platinum kinetics are only useful as a potential measure of toxicity, not antitumor activity. Furthermore, the half-life of cisplatin in serum does not reflect the kinetics in tumor tissue. With respect to the involvement of ABC transporters in drug resistance, we believe that the authors misinterpreted our intent. We presented a brief general discussion of drug resistance, which included the role of various ABC transporters, and the fact that overexpression of these transporters does not appear to play a major role in cisplatin resistance. We never suggested that levels of these transporters should be used as predictive biomarkers of patient response or resistance. Furthermore, as pointed out in our article, resistance to platinum-based chemotherapy is highly complex, and enhanced glutathione efflux, as pointed out by the authors, by overexpression of ABC transporters will not prevent or overcome cisplatin resistance. It is counterintuitive to think that overexpression of the classical ABC drug resistance transporters will result in a durable enhancement of the clinical activity of platinum-based therapies. Finally, we agree that additional in vitro and clinical phenotyping studies are urgently needed to help identify patients at risk of therapy failure. However, until the effects of diagnostic microdosing with respect to the development of drug resistance have been adequately studied, we should not be putting newly diagnosed cancer patients at risk of acquiring resistance by using this technique.