Improving the Therapeutic Index of Navitoclax in the Treatment of Ovarian Cancer

Khaled Alrosan
Keele University

Introduction/Background & aims Ovarian cancer is considered a major health problem. The chemotherapeutic drugs that are used often are inadequate due to the development of drug resistance. The evasion of apoptosis due to overexpression of anti-apoptotic BCL-2 family members is a prominent cause of this resistance. These proteins are also responsible for the progression of the disease. Navitoclax is a BH3 mimetic which inhibits several Bcl-2 family proteins including BCL- X_L, as well as BCL-2. We have previously shown navitoclax potentiates cell death induced by carboplatin. However, clinical success with navitoclax has been limited because it causes mechanism-dependent thrombocytopenia through the inhibition of BCL- X_L in platelets. Unfortunately, ovarian cancer is particularly dependant on Bcl-X_L suggesting that strategies to target navitoclax to ovarian cancer cells may be helpful. Here we have investigated the formulation of navitoclax in nanoparticles to achieve this.

Method/Summary of work Two ovarian cancer cell lines have been chosen including ovcar-8 and OVSAHO. The toxicity for both the encapsulated drug and the drug alone and also with the drug combinations with carboplatin were tested using the trypan blue assay, caspase 3/7, western blot, and annexin v/PI. navitoclax was encapsulated inside the polymer core using probe sonication and the loading capacity was tested using the RP-HPLC. these were followed by measuring the physical characteristics for both the polymer alone and the encapsulated drug. The encapuslated navitoclax was also tested for drug release and its stability was measured using different environmental conditions.

Results/Discussion Navitoclax was encapsulated in nanoparticles comprising a poly(allylamine)-cholesteryl (PAA-Ch₅) micellar core. The nanoparticles provided a 100-fold improvement in the original navitoclax aqueous solubility. Navitoclax (2.5 µM) as a free drug elicited (27 ±8) % death of Ovcar-8 cells and (20 ± 3) % death of Ovsaho cells compared with  (49 ± 8)% and (47 ± 8)% cellular death caused by a similar concentration of navitoclax in the nanoparticles, respectively. Carboplatin (13 µM) caused the death of 28 ± 6% of Ovcar-8 cells and this was augmented by combination with either free navitoclax (1 µM; 62 ± 5% cell death) or by a similar concentration of navitoclax encapsulated in nanoparticles (69 ± 5% cell death). Neither navitoclax (1 µM) nor nanoparticular navitoclax, nor the empty nanoparticles elicited significant cell death. Similar results were observed with Ovsaho cells. These results demonstrate that navitoclax can be effectively encapsulated in nanoparticles and retains its ability to potentiate the cytotoxic activity of carboplatin.

Conclusion(s) There was a successful encapsulation of navitoclax inside the micellar core and improvement in its water solubility. furthermore, its encapsulation does not only improve solubility, but also the small dimensions for the formed nanoparticle can be targeted to the cancer cells passively through EPRE. The formed nanoparticle showed a higher tumour toxicity when compared with the drug alone in addition to its promising synergistic effect with carboplatin.