Proteinuria is a late-onset adverse event in patients treated with cabozantinib

The use of tyrosine kinase inhibitors (TKIs) in thyroid cancer patients is often limited by toxicities. Some have a long-term onset and potentially could impact patients' survival. Among them, there is the nephrotoxicity, mainly represented by proteinuria. The aim of the study was to evaluate the prevalence of proteinuria in medullary thyroid cancer patients treated with cabozantinib, to examine whether it could be a marker for treatment monitoring and to evaluate histological kidney alterations.

We collected data of 31 medullary thyroid cancer patients enrolled in the EXAM trial. Proteinuria was defined and evaluated using the National Cancer Institute's Common Terminology Criteria for Adverse Events. In two symptomatic cases with high-grade proteinuria, a kidney biopsy was performed.

Proteinuria was observed in 4/18 patients (22.2%) and occurred after a mean period of 38 months (median: 35.5 months). It was significantly associated with previous chemotherapy (p = 0.005) and/or treatment with other TKIs (p = 0.04), a prolonged use of cabozantinib (p = 0.0004), and a better radiological response at the end of follow-up (p = 0.002). The kidney biopsy showed pathognomonic features of thrombotic microangiopathy in both cases and a focal amyloid deposit in one.

Proteinuria is a quite frequent adverse event during cabozantinib treatment. It is relatively well manageable with the early detection and correction of risk factors, the temporary discontinuation of cabozantinib and/or its dose reduction, and the use of anti-proteinuric and renoprotective drugs in patient with hypertension. The histological findings confirmed some typical features of the anti-VEGF inhibition injury, already described for other TKIs.