Phase II Window Study of the Farnesyltransferase Inhibitor R115777 (Zarnestra®) in Untreated Juvenile Myelomonocytic Leukemia (JMML): A Children’s Oncology Group Study.

JMML is a rare and often fatal leukemia of young children exhibiting unique clinical, hematopoietic and genetic features including GM-CSF hypersensitivity, and mutations of NF1, RAS, and PTPN11. Ras proteins control a number of cell signaling events becoming activated in part by the addition of a farnesyl moiety via farnesyl protein transferase (FTPase). Given that hyperactive Ras is central to JMML pathogenesis, it is intuitive that an FTPase is an appropriate therapeutic target in JMML. One FTPase inhibitor, L739,749, has previously been shown to abrogate spontaneous in vitro colony growth in 9 JMML samples (Blood 95:639, 2000). R115777 is a potent in vitro and in vivo inhibitor of FTPase, abrogating the growth of H-ras, K-ras and N-ras transformed tumors. In humans, it is well tolerated with the dose-limiting toxicities being myelosuppression and diarrhea. To assess the efficacy and toxicity of R115777 in JMML, a phase II window study was conducted as a part of COG study AAML0122 in newly diagnosed patients who were given the option of receiving this agent prior to cytosine arabinoside, fludarabine and 13-cis retinoic acid followed by stem cell transplant. R115777 was administered PO BID for 21 days with a 7 day rest for two courses in the absence of disease progression or excessive toxicity. The starting dosage in the first 11 patients was 200mg/m² with escalation in subsequent patients to 300mg/m² if the initial dosage was tolerated. Overall response was based upon changes in WBC and organomegaly. The impact of R115777 upon in vitro spontaneous colony growth, GM-CSF hypersensitivity and farnesylation was monitored. A total of 47 patients were accrued: M:F=30:17, median (med) age 15 mos. (1–76); med WBC 30X10⁹/L (4–151); med monocyte count 18X10⁹/L (1–55); med platelet count 58X10⁹/L (2–587); elevated fetal hemoglobin 30 (65%). RAS and PTPN11 mutations were tested in 42 cases and inhibition of prenylation in 33. R115777 was well tolerated at both dosages with the most common grade 3/4 toxicities being thrombocytopenia (40%), anemia (40%), neutropenia (15%), and diarrhea (6%). There were no deaths during the trial. The table details the responses in patients receiving one course (N=47) and 2 courses (N=38) of R115777. The 9 patients not receiving two courses were removed from study due to lack of response or progressive disease.

FTPase activity was inhibited in 13/15 cases (med 71%; range 38–91%) with similar frequency and degree of inhibition at both dosages of R115777. There was no relationship between FTPase inhibition or response and the presence of RAS/PTPN11 mutations or inhibition of prenylation in an HJ2 assay. In conclusion, R115777 provides an overall CR/PR rate of 58% with no significant differences between the two dosages (p=0.7). This agent should be considered in the future management of JMML.