Modulation of death receptor pathways in oncology

Chemotherapeutic efficacy is hampered by occurrence of drug resistance. Several mechanisms cause this phenomenon. A final common factor is the reduced capacity of resistant cells to go into apoptosis following treatment with DNA-damaging agents. It is therefore interesting to search for ways to facilitate this apoptotic process following use of chemotherapeutic drugs. The death receptor ligands tumor necrosis factor (TNF), FasL and TNF-related apoptosis-inducing ligand (TRAIL) might be interesting candidates as they are able to induce apoptosis by binding to their cell membrane receptors. Recombinant forms of these ligands potentiate chemotherapeutic drug effects in preclinical models. For the clinical application of TNF, FasL and TRAIL, it is of primary importance that their safety be guaranteed. RhTNF is the only ligand currently used in humans. However, systemic rhTNF has shown low antitumor activity and higher doses induce severe sepsis-like toxicity. Perfusion setting aimed at limb preservation with rhTNF plus melphalan is currently used in sarcoma patients. A number of options have been tested in the preclinical setting that might allow circumvention of TNF toxicity in the clinic. Systemic rhFasL administration in humans is not yet feasible because of observed severe liver toxicity in mice due to Fas-mediated apoptosis of hepatocytes. Measures to circumvent liver toxicity have not yet been exploited. Another option for using FasL in the clinic may be to identify an alternative route of administration. In the animal model, FasL appeared to be less toxic for the liver compared with anti-Fas antibodies when administered intraperitoneally. There are relatively nontoxic modulators of the Fas death pathway, such as interferon and nonsteroidal antiinflammatory drugs (NSAIDs), which might prove interesting in combination with chemotherapy. Finally, it may be possible to produce a modified FasL with a reduced toxicity profile. TRAIL, produced as soluble, zinc-stabilized rhTRAIL seems to be without preclinical toxicity. Agonistic DR4 and DR5 antibodies against their TRAIL death receptor are being studied as another potential clinical option to induce apoptosis. Due to the synergistic effect observed in the preclinical setting between death receptor ligands and other modulators of the death receptor pathways and chemotherapy, it may well be that this approach is especially of value in the clinic when combined with chemotherapy. Ideally, choices for specific (modified) death receptor ligands for the treatment of patients can be rationally made based on tumor characteristics.