Ritonavir - A Novel Multidrug Resistance Modulator in Cancer Chemotherapy and Ocular Neovascular Diseases
Abstract
Multidrug resistance (MDR), a clinical outcome characterized by subtherapeutic
intracellular drug concentration, is one of the predominant factors limiting effective cancer
chemotherapy. Several possible mechanisms and molecular alterations have been implicated in
the development of MDR, including activation of efflux transporters and metabolizing enzymes
in response to therapeutic agents. Therefore, the primary objective of my dissertation project is to
develop strategies for overcoming drug resistance in cancer chemotherapy. Human
adenocarcinoma cells (LS-180) were treated for 72 hours with vinblastine alone and in the
presence of ritonavir. The expression of efflux transporters (MDR1 and MRP2), metabolizing
enzyme (CYP3A4) and nuclear hormone receptor (PXR) was induced in response to vinblastine.
This overexpression was completely neutralized when cells were cotreated with ritonavir. Uptake
of [3H] lopinavir and Vivid™ assay further confirmed the functional activity of transcribed genes
upon cotreatment. Reduced cell proliferation, migration and increased apoptosis of cancer cells
were further indicative of enhanced activity of chemotherapeutics (doxorubicin, paclitaxel,
tamoxifen and vinblastine) in the presence of ritonavir. Combination therapy of anticancer drug
with ritonavir may overcome drug resistance by neutralizing overexpression of efflux
transporters and metabolizing enzymes.
Hypoxia leading to neovascularization has also been implicated in the development of
MDR and ocular neovascular diseases. Despite introduction of novel therapeutics, treatment of retinal disorders remains challenging, possibly due to complex nature of hypoxia signaling. This
study demonstrates for the first time that hypoxic conditions may alter expression of efflux and
influx transporters in retinal pigment epithelial (RPE) cells. These findings suggest that hypoxia
may further alter disposition of ophthalmic drugs. Inhibiting this signaling mechanism with an
already approved therapeutic molecule may have promising antiangiogenic role with fewer side
effects. Our studies (quantitative PCR, immunoblot analysis, ELISA and angiogenic assay) have
demonstrated that ritonavir inhibits the expression of hypoxia-inducible factor-1α (HIF-1α)
mediated vascular endothelial growth factor (VEGF) expression in RPE cells probably via
inhibition of PI3K/AKT pathway. This inhibition may reduce retinal neovascularization. These
findings shed new light on the possibility of incorporating ritonavir in the treatment regimen of
ocular angiogenic diseases. Although many inhibitors of HIF-1α are in clinical trials, additional
benefit of using ritonavir is that it has been given to HIV patients with relatively low toxicity.
The process of traditional drug development could be fast tracked since ritonavir is clinically
approved for human use. However, further preclinical and clinical experiments are necessary to
determine the repositioning of ritonavir in the treatment of ocular neovascular diseases
Table of Contents
Mechanisms of drug resistance in cancer chemotherapy: Coordinated role and regulation of efflux transporters and metabolizing enzymes -- Differential effect of MDR1 and MRP2 in cellular translocation of gemifloxacin -- PXR mediated induction of efflux transporters by fluoroquinolones: a possible mechanism for development of multidrug resistance -- Rotonavir: a novel therapeutic for overcoming drug resistance in cancer chemotherapy -- Hypoxia-inducible factor 1 (HIF-1): a potential target intervention in ocular neovascular diseases -- Molecular expression and functional activity of efflux and influx transporters in hypoxia induced retinal pigment epithelial cells -- Ritonavir inhibits HIF-1a mediated VEGF expression in retinal pigment epithelial cells -- Summary and recommendations -- Appendix