Comparative neurotoxicity of oxaliplatin, ormaplatin, and their biotransformation products utilizing a rat dorsal root ganglia in vitro explant culture model

Abstract

Purpose: Neurotoxicity is one of the major toxicities of platinum-based anticancer drugs, especially oxaliplatin and ormaplatin. It has been postulated that biotransformation products are likely to be responsible for the toxicity of platinum drugs. In our preceding pharmacokinetic study, both oxaliplatin and ormaplatin were observed to produce the same types of major plasma biotransformation products. However, while the plasma concentration of ormaplatin was much lower than that of oxaliplatin at an equimolar dose, one of their common biotransformation products, Pt(dach)Cl₂, was present at 29-fold higher concentrations in the plasma following the i.v. injection of ormaplatin than of oxaliplatin. Because ormaplatin has severe neurotoxicity and Pt(dach)Cl₂ is very cytotoxic, we have postulated that Pt(dach)Cl₂ is likely to be responsible for the differences in neurotoxicity between ormaplatin and oxaliplatin. In order to test this hypothesis, we compared the neurotoxicity of oxaliplatin, ormaplatin, and their biotransformation products. Since the dorsal root ganglia (DRGs) have been suggested to be the likely targtet for platinum drugs and in vitro DRG explant cultures have been suggested to be a valid model for studying cisplatin-associated neurotoxicity, our comparative neurotoxicity study was conducted with DRG explant cultures in vitro. Methods: Based on the previous studies of cisplatin neurotoxicity, we established our in vitro DRG explant culture utilizing DRGs dissected from E-19 embryonic rats. Rat DRGs were incubated for 30 min with different platinum compounds to mimic in vivo exposure conditions; this was by followed by a 48-h incubation in culture medium at 37 °C. At the end of the incubation, the neurites were fixed and stained with toluidine blue, and neurite outgrowth was quantitated by phase-contrast microscopy. The inhibition of neurite outgrowth by platinum compounds was used as an indicator of in vitro neurotoxicity. Since an in vivo study has indicated that the order of neurotoxicity is ormaplatin > cisplatin ≥ oxaliplatin > carboplatin as measured by morphometric changes to rat DRGs, we initially validated our DRG explant culture model by comparing the in vitro neurotoxicity of ormaplatin, cisplatin, oxaliplatin, and carboplatin. After observing the same neurotoxicity rank between this study and a previous in vivo study, we further compared the neurotoxicity of oxaliplatin, ormaplatin, and their biotransformation products including Pt(dach)Cl₂, Pt(dach)(H₂O)Cl, Pt(dach)(H₂O)₂, Pt(dach)(Met), and Pt(dach)(GSH) utilizing the DRG explant culture model. Results: Our study indicated that Pt(dach)Cl₂ and its hydrolysis products were more potent at inhibiting neurite outgrowth than the parent drugs oxaliplatin and ormaplatin. In contrast, no detectable inhibition of neurite outgrowth was observed for DRGs dosed with Pt(dach)(Met) and Pt(dach)(GSH). Conclusion: This study suggests that biotransformation products such as Pt(dach)Cl₂ and its hydrolysis products are more neurotoxic than the parent drugs oxaliplatin and ormaplatin. The different neurotoxicity profiles of oxaliplatin and ormaplatin are more likely due to the different plasma concentrations of their common biotransformation product Pt(dach)Cl₂ than to differences in their intrinsic neurotoxicity.