Modulation of anticancer agents in the clinic: Challenges and opportunities
Authors: Miguel Villalona-Calero
Journal: Clinical Cancer Research (2008)

PL01-03   The excitement produced by the plethora of anticancer molecular targeted therapies available for clinical trials has caused an open disregard by many clinicians and scientists for the “not sexy”, “not intelligent” and also “toxic” conventional chemotherapy. It is surprising the tendency to forget that chemotherapy is the only systemic treatment that has been consistently demonstrated to result in cures, both in localized and extensive disease. Advances in technology have permitted a better understanding of the mechanisms of action and metabolism of many natural products derivatives currently approved for human cancer treatment. This knowledge suggests that chemotherapy can be as sexy and as smart as the approach we utilize to extract its maximum benefit. The opportunity exists to: 1- use conventional chemotherapeutic drugs in combination to enhance their therapeutic index, 2- target specific patient populations with cancers that are exquisitely susceptible to the specific chemotherapy agent and 3- to use molecularly targeted agents to increase chemotherapy9s overall efficacy.   1- Modulating chemotherapy with chemotherapy  The oral fluoropyrimidine prodrug capecitabine has been an attractive drug to modulate. It was designed to avoid elimination in the gastrointestinal tract and plasma by dyhidropirimidine dehydrogenase, through a clever multistep enzymatic metabolism which involves hepatic carboxylesterases and hepatic and extrahepatic cytidine deaminases. The resulting plasma metabolite, doxifluridine is further converted into the active 5-fluorouracil by the enzyme thymidine phosphorylase (dThdPase), a potent tumor associated angiogenesis factor, preferentially expressed in tumor tissue. Based on preclinical evidence of upregulation of dThdPase by paclitaxel and docetaxel, we and others conducted studies of combination of capecitabine with these agents in several solid malignancies with very encouraging antitumor activity. A large randomized trial in breast cancer established docetaxel/capecitabine as one of the standard regimens of use for the treatment of this disease.  Irinotecan is a semisynthetic analog of camptothecin, which interacts with cellular Topo 1-DNA complexes and produces S-phase specific cytotoxicity. Irinotecan is subject to extensive metabolic conversion by various enzymatic systems. Carboxylesterases convert it into its more active metabolite SN-38, while inactivation to SN38G occurs by glucoronidation by UGT1A1. Irinotecan9s complex metabolism makes this agent a target for interaction with other drugs. Enzyme-inducing antiepileptic drugs, for example, increase P450 CYP3A4 mediated conversion to the inactive APC metabolite. Conversely, valproic acid inhibits SN-38 conjugation decreasing irinotecan’s clearance. Because Topo-1 is the cellular target of irinotecan, it is conceivable that its cellular level and activity would be proportional to irinotecan cytotoxic effects. In fact, synergism was observed when irinotecan was combined with mitomycin-C (MMC) in cell lines culture. The mechanism responsible for this interaction was an increase in Topo-1 activity, as measured by relaxation of supercoiled DNA, following exposure to MMC.  Another mechanism of irinotecan resistance is activation of the transcription factor NFkB (nuclear factor kB) by DNA damage after irinotecan treatment. NFκB upregulates the expression of a number of pro-proliferative and anti-apoptotic genes in the cell. Inhibition of NFκB activation by intracellular introduction of the super-repressor [IκBα] significantly increased SN-38 induced cytotoxicity in cell lines and xenografts. Both thalidomide and cyclooxygenase (Cox) inhibitors have been shown to inhibit NFκB activation by specific binding and inactivation of the enzyme that leads to the nuclear translocation of NFκB.  We performed feasibility trials of the combination of irinotecan and low doses of MMC and of irinotecan plus thalidomide, as well as double modulation of irinotecan with MMC/celecoxib and thalidomide/celecoxib in patients with refractory malignancies. Antitumor activity was demonstrated by the combination of irinotecan and MMC (lung, esophageal and breast cancers). Phase II trials already completed in esophageal and breast cancer patients showed prominent activity. The combination with thalidomide confirmed the observation of others of some amelioration of irinotecan’s toxicity and the observation of antitumor activity in chemotherapy refractory patients, in the absence of pharmacokinetic interactions. Of interest is that double modulation resulted in unacceptable toxicity for irinotecan/MMC/celecoxib, and a lack of improvement in tolerability for irinotecan/thalidomide/celecoxib compared to irinotecan/thalidomide.   2- Focusing chemotherapy treatment to repair-deficient cancers  Potential elimination of micrometastatic disease is a goal of adjuvant chemotherapy treatment, and improvements in survival are clearly documented. However, a good number of patients are exposed to unnecessary toxicities. Soria and collaborators focused on the expression of the excision repair cross-complementation group 1 (ERCC1) protein in operative specimens of non small cell lung cancer. Using a large tumor bank from patients enrolled in the international adjuvant lung cancer trial (IALT), a positive study that produced a large impact in chemotherapy prescribing practices, they showed ERCC1 expression in roughly half of the tumors available. Their study demonstrated a significant benefit of platinum therapy in patients that were ERCC1 deficient and a possible negative impact in those with ERCC1 positive tumors. Similarly, it has been hypothesized that patients with tumors deficient in ribonucleotide reductase 1 (RRM1), an enzyme that plays a large role in the metabolism of gemcitabine, should be more susceptible to the antitumor action of this agent. Prospective clinical trials are ongoing, in which allocation to platinum or gemcitabine based therapy is done according to tumor expression of ERCC1 and RRM1.  Important to DNA repair are the genes involved in the Fanconi anemia (FA)/BRCA pathway. Germline defects result in a disease characterized by developmental abnormalities, bone marrow failure, and cancer predisposition. Study of the genotype of this disease reveals 13 complementation groups. Eight of these proteins are subunits of an FA core complex, which activates FancD2 by monoubiquitinating the protein as a response to DNA damage. Activated FancD2 is then transported to subnuclear foci thought to be the sites of DNA repair and which also contain BRCA1, FancD1(BRCA2) and Rad51. Mutations that disrupts components of the core complex abrogate the E3 ligase function of the complex, leading to defective FancD2 monoubiquitination and lack of foci formation. Disruptions in the aggregation of BRCA1, BRCA2 and Rad51 also interfere with foci formation and repair.  Given the essential role of this pathway in DNA repair, FA/BRCA pathway-defective cancers are expected to be sensitive to DNA damage inducing agents. In fact, neoplasms in individuals with germline defects of this pathway are exquisitively sensitive to DNA breaking agents and agents that disrupt alternative repair mechanisms, for example poly(ADP-ribose) polymerase (PARP) inhibitors, have been shown to have single agent activity in patients with BRCA deficient cancers. It is conceivable, that somatic alterations in this pathway throughout diverse malignancies may not be uncommon. For these cases the therapeutic index of DNA breaking agents would be considerably increased, especially when alternative repair mechanisms are targeted as well.   3- Modulating chemotherapy with molecularly targeted agents.   High expectations have been placed on the potential synergistic effects of chemotherapy and molecularly targeted agents when combined. Initial success was demonstrated with trastuzumab. Patients selected for amplification of her2 (ERBB2) appear to derive additional clinical benefit when trastuzumab is combined with chemotherapy. The clinical benefit, although not agent specific, appear optimal for taxanes and platinums, while combination with anthracyclines were too taxing on heart function, making the combination unrealistic in the clinic. Synergistic effect was also predicted for her1 (EGFR) inhibitors when combined with chemotherapy. This appears partially true for antibodies, but transmembrane acting small molecule tyrosine kinase inhibitors have been a big disappointment when combined with chemotherapy. It is possible that the failure of this approach has been due to the lack of population enrichment featured in the trastuzumab trials. Studies being conducted in patients with tumor EGFR amplification or in non smokers may clarify this issue.  It is still unexplained however, the partial success of combining anti EGFR antibodies with chemotherapy, and how ubiquitously expressed is EGFR, suggesting that this gene is important not only for the “kinase-addicted” tumors. An interesting recent observation is the function of kinase-independent EGFR to prevent autophagic cell death by maintaining intracellular glucose levels by stabilization of the sodium/glucose co-transporter. This function is clearly not affected by tyrosine kinase inhibitors, although it is yet unclear if antibodies may have the ability to do so. Effective inhibition of this function open the opportunity for increase susceptibility to chemotherapy insult.  Bevacizumab appears to increase the antitumor effect of chemotherapy, while small molecule VEGF kinase inhibitors have not been to date effectively combined. Improved tumor delivery of chemotherapy through a larger window for the longer half-life antibodies may mediate this differential effect.