Resolution of Hairy Cell Leukemia Minimal Residual Disease by Both BRAF and Clone-Specific Real-Time Quantitative PCR (RQ-PCR) After Treatment with Moxetumomab Pasudotox.

Moxetumomab pasudotox is an anti-CD22 recombinant immunotoxin containing truncated Pseudomonas exotoxin which was recently reported to achieve a complete remission rate of 46% in 28 patients with relapsed/refractory hairy cell leukemia (HCL). An additional 20 patients were treated at the highest dose level and are now fully evaluable for response and minimal residual disease (MRD) determinations. RQ-PCR using clone-specific primers and a clone-specific TaqMan probe is capable of detecting one HCL cell in 10⁶normal cells. Recently reported methods to detect the HCL-associated BRAF V600E mutation include pyrosequencing (5–10% sensitivity) and PCR (0.1–0.23% sensitivity).

Moxetumomab pasudotox was administered to 16 patients at 5–40 ug/Kg every other day for 3 doses (QODx3) and to 32 patients at 50 ug/Kg QODx3, via 1–16 (median 4) cycles per patient at 4-week intervals. Complete remission (CR) required resolution of cytopenias and elimination of HCL in the blood and marrow by standard microscopy, but MRD could be present by flow cytometry of blood or bone marrow aspirate (BMA) or immunohistochemistry (IHC) of the bone marrow biopsy (BMBx). Blood and marrow from patients were also tested by PCR using consensus primers. When immunoglobulin (Ig) rearrangements could be cloned, RQ-PCR using clone-specific primer and probe was performed. To detect MRD by the BRAF V600E mutation, BRAF quantitative PCR (BRAF-qPCR) was performed on cDNA samples, using mutant-specific primer, and SYBR-Green detection followed by melting point analysis. MRD testing for BRAF-qPCR, unlike clone-specific RQ-PCR, did not require prior cloning of the Ig rearrangement.

All 198 cycles of moxetumomab administered to 48 patients were evaluable for toxicity and response. No dose limiting toxicity was observed, although 2 patients as previously reported had a grade 2 hemolytic uremic syndrome with transient grade 1 platelet and creatinine abnormalities. Of the 48 HCL patients at all dose levels, there were 26 (54%) CRs, with an overall response rate (ORR) of 88%. Of 32 at 50 ug/Kg QODx3, there were 19 (59%) CRs with an ORR of 91%. Of these 19 CRs, 11 (58%) achieved MRD negativity by repeated flow cytometry of both BMA and blood and IHC of BMBx. Flow cytometry of the BMA was the most sensitive conventional test of MRD. Of the 9 CRs at 50 ug/Kg QODx3 evaluable by clone-specific RQ-PCR of blood, 5 negative were also flow-negative, and 4 positive were also flow-positive (p=0.008). BRAF-qPCR on cDNA from limiting dilutions of BRAF V600E+ Colo-205 cells into BRAF wild-type cells achieved consistent detection at 1:10⁵dilution (0.001%). Of 10 flow-negative CRs at 50 ug/Kg QODx3 evaluated by BRAF-qPCR, all 10 (100%) were BRAF-qPCR negative, including 4 which were nonevaluable by RQ-PCR due to inability to clone the Ig rearrangements prior to treatment. Currently 12 (63%) of the 19 CRs at 50 ug/Kg QODx3 are ongoing at 6–47 (median 21) months, including 10 (91%) of 11 MRD-negative vs 2 (25%) of 8 MRD+ CRs (p=0.006).

Moxetumomab pasudotox is active in relapsed and refractory HCL and has a safety profile supporting further development for this disease. Retreatment on this trial could not necessarily be extended to achieve MRD-negative BMAs or molecular remission by RQ-PCR using sequence-specific or BRAF primers. However, these tests might be useful in the future to guide retreatment, optimize CR durability and possibly eradicate the HCL clone in selected patients. This summary contains investigator reported data. This study was sponsored by MedImmune, LLC, and supported by NCI's Intramural Research Program and the Hairy Cell Leukemia Research Foundation.

Kreitman:NIH: Co-inventor on the NIH patent for Moxetumomab Pasudotox, Co-inventor on the NIH patent for Moxetumomab Pasudotox Patents & Royalties. Off Label Use: Moxetumomab Pasudotox is an experimental agent for CD22+ hematologic malignancies. FitzGerald:NIH: Coinventor on the NIH patent for Moxetumomab Pasudotox, Coinventor on the NIH patent for Moxetumomab Pasudotox Patents & Royalties. Fei:AstraZeneca: Stock, Stock Other; MedImmune, LLC: Employment. Ibrahim:AstraZeneca: Stocks, Stocks Other; MedImmune: Employment. Pastan:NIH: Coinventor on NIH patent for moxetumomab pasudotox, Coinventor on NIH patent for moxetumomab pasudotox Patents & Royalties.