Lawrence D. Mayer

Molecular and pharmacological strategies to overcome multidrug resistance

Multidrug resistance is a major obstacle to the effective treatment of cancer. Despite vast improvements in our understanding of the mechanisms of drug resistance, relatively few significant advances have been made towards effectively circumventing it in a clinical setting. The ability to modulate multidrug resistance has been complicated by the fact that many human tumors simultaneously exhibit multiple resistance mechanisms. In order to effectively overcome multidrug resistance it will be necessary to design new strategies that combine multiple modulating agents and approaches. This review provides an overview of the major causes of multidrug resistance and summarizes many of the current approaches being taken to overcome it. We also describe how liposomal drug delivery systems can be utilized to aid in achieving these goals.

The role for liposomal drug delivery in molecular and pharmacological strategies to overcome multidrug resistance.

When P-glycoprotein (PGP) was first identified as a direct mediator of multidrug resistance (MDR) a great deal of excitement was generated as scientists and clinicians anticipated the ability to successfully treat previously refractory cancers by blocking this drug efflux pump. More than twenty years later there is still minimal evidence that inhibiting PGP will have widespread impact on the chemosensitivity of human tumors. Yet, we know that PGP is over-expressed in many cancers, is associated with poor prognosis in certain tumor types and, if functional, will certainly reduce the accumulation of many common anticancer drugs inside tumor cells exhibiting elevated PGP levels. Similar situations have arisen more recently for other potential mediators of chemosensitivity such as the apoptosis antagonist protein Bcl-2. Bcl-2 has been linked to drug resistance and poor patient prognosis in numerous studies. There has been a great deal of interest in blocking expression or function of this protein to increase the susceptibility of tumor cells to apoptotic stimuli such as chemotherapy. However, preclinical and clinical evidence supporting this approach as a unilateral means of significantly enhancing the response of tumors to chemotherapy is limited. In view of these examples, it would appear likely that similar caveats will be experienced in the future as new molecular targets are identified for potential MDR reversal.Given the ever increasing evidence of genetic diversity in cancer development and progression, it should not be surprising that the development of MDR is also complex and heterogeneous. Consequently, it should also not be surprising that solutions to this problem are unlikely to arise from interventions aimed at any single resistance mechanism. These concepts suggest that new approaches to addressing the various molecular and pharmacological features associated with MDR will be necessary in order to make significant in-roads into improving the clinical activity of current and future anticancer agents. This review summarizes many of the current directions being taken to overcome MDR and how liposomal drug delivery systems may play an important role in achieving this aim.

Drug interaction studies between paclitaxel (Taxol) and OC144-093 — A new modulator of MDR in cancer chemotherapy

SummaryThe MDR modulator, OC144-093, is a potential candidate for use in cancer therapy and exhibits potent biological activityin vitro andin vivo when combined with anticancer agents such as paclitaxel [1]. Its inhibitory interaction with P-glycoprotein (Pgp), the mdr1 gene product and a mechanistic participant in multidrug resistance [2], underlies its activity as a modulator of MDR. Having previously shown that OC144-093 is not a substrate for CYP3A [4] we first examined the effects of OC144-093 on paclitaxel metabolismin vitro. Using human liver microsomes, we have demonstrated that OC144-093 inhibited the CYP3A mediated metabolism of paclitaxel at high concentrations only (Ki=39.8±5.1 μM, n=3). Pharmacokinetic results also show that an oral dose of OC144-093, co-administered with paclitaxel caused negligible disturbance of the pharmacokinetic profile for paclitaxel when injected intravenously. In contrast, AUC values were elevated approximately 1.5-fold in all groups treated orally with paclitaxel and OC144-993. Cmax was enhanced approximately 2-fold in the co-dosed group. These characteristics are consistent with Pgp blockade in the gut enhancing oral bioavailability. Elimination properties of paclitaxel were affected only upon multiple dosing of OC144-093. These results warrant the further clinical assessment of OC144-093 as an MDR reversing agent.

CHAPTER 4.2 – Designing therapeutically optimized liposomal anticancer delivery systems: Lessons from conventional liposomes

This chapter describes liposomal anticancer drugs, and the dilemma faced when designing optimized liposomal anticancer drugs. Recent technological advances in the production, stability, and biological properties of liposomes have greatly increased the degree of sophistication that can be designed into liposomes to improve their therapeutic/toxic activity profile. Investigators designing liposomal anticancer drug carrier technology must contend with a dilemma of opposing goals in the different biological compartments that the formulations experience. Because the uptake of liposomes in tumors appears to be passive, extended circulation times appear necessary to facilitate liposome accumulation. It follows that drug leakage from the liposomes must be minimized to avoid toxicities associated with free drug. The inability to differentially control drug release rates in the plasma compartment and disease site is perhaps the most significant limitation of presently available liposomes. The processes that dictate the fate of liposomes after intravenous injection has increased and help to design formulations that will optimize the selectivity of action for encapsulated agents. Inclusion of additional components into conventional liposomes can now be done on the basis of extensive data describing the in vivo behavior of various liposome types.

Assessment of the involvement of CYP3A in the vitro metabolism of a new modulator of MDR in cancer chemotherapy, OC144-193, by human liver microsomes

SummaryThe novel substituted imidazole compound, OC144–093 exhibits potent biological activity in vitro and in vivo for reversal of P-glycoprotein (PgP) based resistance to cancer chemotherapy [1]. Its mechanism of action relies upon its inhibitory interaction with the mdr1 gene product, a known mediator of multidrug resistance (MDR) [2]. Overlapping substrate specificities and tissue distribution of cytochrome P450 3A (CYP3A) and PgP indicate the potential for drug-drug interactions when modulator and anticancer agent are coadministered [3]. We have examined the metabolism of OC144–093 in vitro using human liver microsomes to determine if CYP3A is involved. Our results show that OC144–093 is converted to one major metabolite (M1) in human liver microsomes which was identified by LCMS to be the O-deethylated derivative. Km and Vmax for O-deethylation were determined as 3.96±0.67 μM and 32.08±9.73 pmol/mg protein/min, respectively (n=3). Correlation studies conducted in a panel of human livers phenotyped for specific P450 enzyme activity showed a significant relationship between M1 formation and the activity of CYP2C9, CYP2B6, CYP2E1 and CYP3A4. Treatment of microsomes with carbon monoxide gas inhibited M1 formation and diethyldithiocarbamate and ketoconazole (>3 μM), non-specific CYP inhibitors, gave IC50 values of 124.4±21.6 μM and 25.3±3.2 μM respectively for the inhibition of O-deethylation also implicating the involvement of CYP enzymes. Specific CYP inhibitors of CYP3A4 were essentially non-inhibitory to M1 formation. We can conclude therefore that OC144–093 is not extensively metabolised in human liver microsomes although conversion to its O-deethylated derivative does occur. Our data indicates that this conversion is not mediated by CYP3A4.

Versatile Fixed-Ratio Drug Combination Delivery Using Hydrophobic Prodrug Nanoparticles

The current paradigm of cancer chemotherapy involves the co-administration of multiple anticancer agents at their maximum tolerated doses to achieve greater antitumor activity than could be realized with single agents alone. Emerging evidence, however, points to the important role drug ratios play in determining whether in vivo drug interactions are synergistic or antagonistic in nature. The CombiPlex® technology platform was developed to deliver multiple chemotherapy drugs at a defined synergistic drug ratio via a particulate carrier. The promising clinical results of CPX-351 in the treatment of newly diagnosed acute myelogenous leukemia (AML) serve as an example of the magnitude of gains that can be made when combination chemotherapy drugs are delivered to the target site at their synergistic ratio. While the CombiPlex technology had been used to develop three clinical and preclinical liposomal products, there have been several reports of drug combinations formulated into polymer-based nanoparticles, typically involving hydrophobic drugs which are not generally suitable for liposome encapsulation.