Summary and Discussion

SUMMARY This thesis deals with several clinical and preclinical aspects of novel microtubule-stabilizing agents in the treatment of solid tumors. Chapter 1 gives an introduction on the current position of microtubule-stabilizing agents in the treatment of malignant diseases and their possible routes of administration. In addition, an outline is given of the molecular mechanisms leading to chemotherapy-induced cell death in human cancer cells. The first part of the thesis focuses on preclinical aspects of chemotherapeutic drugs and microtubule-stabilizing agents in particular. Despite its achievements, the effectiveness of chemotherapy is still hampered by the development of drug resistance. Chapter 2 gives an overview of the various molecular pathways leading to multidrug resistance, and a series of attempts to manipulate these pathways are discussed. These efforts are, however, limited by the coexistence of multiple, partially unidentified resistance mechanisms in human cancer. Recently, progress has been made in the field of gene sequencing and microarray technologies and these techniques could be helpful to identify genetic abnormalities in individual human tumors that could be relevant for predicting response. In Chapter 3 we investigated the contribution of the major apoptotic pathways to the cytotoxic effects discodermolide and epothilone B, two novel microtu-bule-interacting agents with potent in vivo anti-tumor activity. Despite late activation of the apoptotic machinery, neither blockade of the mitochondria nor death receptor pathways did substantially reduce the cytotoxic effects of these drugs. Based on these findings, we hypothesized that the observed activation of the apoptotic machinery occurs only as a bystander effect and is not relevant for the cytotoxic effects of discodermolide and epothilone B. In support of this view, preincubation with the broad-spectrum caspase blocker zVAD-fmk, did not decrease the effects of these compounds. Our findings imply that caspase-independent routes are involved in the cytotoxic effects of discodermolide and epothilone B. Caspase-independent cell death pathways induced by mircotubule-stabilizing agents are further studied in Chapter 4. Here we present two lines of evidence indicating a central role for the lysosomal protease cathepsin B in mediating cell death by discodermolide, epothilone B and paclitaxel. First, inhibition of cathepsin B, and not of caspases or other proteases, such as cathepsin D or calpains, results in a strong protection against drug-induced cell death in several NSCLC cells. Second , these microtubule-stabilizing agents trigger disruption of lysosomes and release and activation of cathepsin B. Interestingly, inhibition of cathepsin B prevents the appearance of multinucleated cells, an early …