Abstract 1363: Exposure-toxicity analysis of cabozantinib in patients with salivary gland cancer and renal cell cancer

Abstract Background Cabozantinib is approved for treatment of renal cell cancer (RCC) in a starting dose of 60 mg. However, in the registration studies dose reductions were needed in 46-62% of patients due to toxicity. Improved clinical efficacy of cabozantinib was observed in RCC patients with an average exposure of > 750 ug/L. Data about cabozantinib pharmacokinetics in patients routinely treated with this drug is scarce, while treatment is challenging due to toxicity. Therefore, we explored the cabozantinib exposure in patients with two different tumour types in our clinic. Methods Clinical data and cabozantinib trough concentrations (Cmin) were collected from all patients treated with cabozantinib with at least one measured Cmin at steady state in our clinic between Jan 2018 - Aug 2020. We included salivary gland cancer (SGC) patients treated in a phase II study and RCC patients from our outpatient clinic. Geometric mean (GM) Cmin at the start dose, at 40 mg and at best tolerated dose (BTD) were compared between the two tumour types. Results In total 47 patients were included. All patients with SGC (n=22) started with 60 mg, while 13 of 25 patients with RCC started with 40 mg. GM Cmin level at the start dose was 1350 µg/L (95% CI 1182-1781) vs. 689 µg/L (95% CI 576-823) (P<.001) for patients with SGC and RCC, respectively. When dose-normalized to 40 mg, SGC patients had a significantly higher cabozantinib exposure compared to RCC patients (GM Cmin 968 µg/L (95% CI 788-1188) vs. 676 µg/L (95% CI 571-799)(P=.007). A higher cabozantinib GM Cmin level was observed in female compared to male patients. Dose reductions due to toxicity were needed in 91% and 60% of patients with SGC and RCC, respectively. Median BTD was 25 mg for SGC patients and 40 mg for RCC patients. GM Cmin at BTD were comparable between the SGC group and the RCC group, i.e. 692 µg/L (95% CI 582-822) vs. 576 µg/L (95% CI 493-673) (P=.1), respectively. At BTD, 64% of SGC patients and 80% of RCC patients had an exposure below the proposed target of >750 ug/L. Conclusion Unexpectedly, cabozantinib levels were significantly higher in patients with SGC compared to those with RCC at the dose of 40 mg. However, cabozantinib levels at BTD were comparable between patients with SGC and RCC. At BTD, the majority of patients did not reach the target of >750 µg/L. For most patients with RCC, the cabozantinib level at BTD corresponded to a dose of 40 mg. Therefore, 40 mg instead of 60 mg as a starting dose followed by dose-adjustment based on exposure and tolerability may be preferred in patients with RCC. Although cabozantinib is not registered for SGC, an even lower starting dose of 20 mg/day may be required in these patients based on cabozantinib levels at BTD. Future studies should focus on identifying an optimal and tolerable exposure-response target value for cabozantinib and elucidate factors that contribute to the differences in exposure, in order to individualise and improve treatment. Citation Format: Stefanie D. Krens, Wim van Boxtel, Maike J. Uijen, Frank G. Jansman, Ingrid M. Desar, Sasja F. Mulder, Carla M. van Herpen, Nielka P. van Erp. Exposure-toxicity analysis of cabozantinib in patients with salivary gland cancer and renal cell cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1363.