Paton, Emily L.

ABSTRACT Introduction Hairy cell leukemia (HCL) is an uncommon hematopoietic stem cell disease known to have an underlying somatic BRAF V600E driver mutation. Currently, the most accepted treatment is five or seven consecutive days of outpatient infusion of cladribine (e.g., continuous infusion), despite frequent antibiotic use and hospitalization with this regimen. As a result, a few investigators have adopted weekly infusion of cladribine with similar or improved results and fewer side effects. Methods We conducted a retrospective, single‐institution cohort study of 36 treatment‐naïve patients with HCL treated at the University of Colorado Hospital. Eighteen patients received intermittent weekly cladribine (intermittent cladribine [IC]) for 5–7 doses and 18 received standard daily cladribine (continuous cladribine [CC]) over 5–7 days. We report clinical cohort characteristics, response rates, progression‐free survival, overall survival, toxicity, hospitalization rates, and measurable residual disease monitoring in select patients using peripheral blood quantitative BRAF V600E PCR. Results We report here our experience with 18 patients with newly diagnosed BRAF ‐mutated HCL treated with weekly cladribine at a single institution compared to 18 HCL patients who received continuous infusions. Baseline hematologic parameters and overall survival were comparable between groups ( p = 0.1135), but there was a higher rate of complete remission for patients treated with IC (94.4% vs. 61.1%) with significantly improved progression‐free survival ( p < 0.0001). We observed comparable rates of neutropenia, neutropenic fever, antibiotic use, and G‐CSF administration. There were fewer hospitalizations for patients treated with IC (4/18) compared to patients treated with CC (6/18). Rituximab exposure differed between groups, with 72% of patients in the IC cohort receiving rituximab compared with 27.2% in the CC cohort (although treatment status was documented for only 66.7% of patients in the CC cohort). Peripheral blood BRAF V600E allele burden significantly declined and correlated with clinical remission, but this was only performed in select patients who were treated in recent years. Conclusions In this retrospective analysis, IC dosed weekly was associated with improved progression‐free survival and higher rates of complete remission compared to CC with comparable toxicity and fewer hospitalizations. These findings suggest a potential clinical benefit from this regimen. However, interpretation of these findings is limited by the non‐randomized design and differences in rituximab exposure between groups, which may have contributed to the observed outcomes and limited definitive conclusions. Trial Registration: The authors have confirmed clinical trial registration is not needed for this submission