Practical Considerations for Treatment of Relapsed/Refractory FLT3-ITD Acute Myeloid Leukaemia with Quizartinib: Illustrative Case Reports.

Quizartinib is a tyrosine kinase inhibitor selectively targeting the FMS-like tyrosine kinase 3 (FLT3) receptor that has been developed for the treatment of acute myeloid leukaemia (AML). The Phase 3 QuANTUM-R study investigated the efficacy of quizartinib monotherapy in patients with relapsed/refractory FLT3-ITD mutation-positive AML. The clinical course of four QuANTUM-R participants exemplifies issues specific to quizartinib treatment and is described here. Patient 1 was FLT3-ITD mutation-negative at AML diagnosis, but became FLT3-ITD mutation-positive during treatment that included several lines of chemotherapy and was therefore a suitable candidate for quizartinib. Because of the clonal shifts of AML during treatment, retesting genetic alterations at each relapse or resistance may help to identify candidates for targeted treatment options. Patient 2 developed QTc prolongation during quizartinib treatment, but the QTc interval normalised after dose reduction, allowing the patient to continue treatment and eventually resume the recommended dose. Patient 3 responded to quizartinib and was scheduled for haematopoietic stem cell transplant (HSCT), but developed febrile neutropenia and invasive aspergillosis during conditioning and subsequently died (to avoid drug-drug interactions, no azole antifungal was administered concomitantly). Care is required when selecting concomitant medications, and if there is potential for interactions (e.g. if prophylactic azole antifungals are required) the quizartinib dose should be reduced to minimise the risk of QTc prolongation. Patient 4 was able to undergo HSCT after responding to quizartinib and experienced a durable response after HSCT while on quizartinib maintenance therapy. Together, these cases illustrate the main issues to be addressed when managing patients under quizartinib, allowing for adequate scheduling and tolerability, bridging to HSCT, and durable remission on maintenance therapy in some patients.