Initial Safety, Pharmacokinetic and Pharmacodynamic Data from a Phase I Clinical Trial of Systemic C-MYB Antisense Oligodeoxynucleotide in Subjects with Refractory Hematologic Malignancies

BACKGROUND: Conventional treatment options for patients with relapsed hematologic malignancies are both limited and highly toxic driving the pursuit of more tumor specific and less toxic therapies. RNA targeted oligonucleotides are potentially powerful drugs with the ability to silence genes required for malignant hematopoietic cell growth at the post transcriptional level. Studies from our laboratory have validated the c-myb proto-oncogene, which regulates important hematopoietic cell functions and is overexpressed in many hematologic malignancies, as a target for this technology. A prior Phase I trial using a 24 nucleotide phosphorothioated antisense oligodeoxynucleotide targeted to c-Myb mRNA (C-MYB AS ODN) did not identify a maximum tolerated dose (MTD) of the drug. Here we report initial results of a follow up Phase I dose escalation trial using C-MYB AS ODN at higher dose levels than previously studied in subjects with refractory hematologic malignancies.

METHODS: C-MYB AS ODN is administered as a 7 day continuous infusion. 5 dose levels ranging from 3mg/kg/day to 12mg/kg/day are planned. Subjects are enrolled using an accelerated dose escalation scheme in which one subject is enrolled on each dose level (DL) with plans to revert to the standard 3+3 design in the event of significant attributable toxcity. C-MYB AS ODN concentrations are measured in peripheral blood (PB) and in mononuclear cells (MNC) by slot blotting at baseline, days 3 and 7 of infusion and two weeks after cessation of infusion. C-myb expression is assessed at these timepoints though QRT-PCR for c-myb RNA. Disease specific assessments of response are measured at predefined timepoints after therapy.

RESULTS: 6 subjects, all with refractory acute myelogenous leukemia (AML) have enrolled to date. Escalation through the first 3 DLs occurred without any toxicities. At DL4 (10mg/kg/day) abnormalities have been noted in coagulation assays. The first subject enrolled on DL 4 developed a grade 3 prolongation of the activated partial thromboplastin time (PTT) attributable to drug which returned to normal within 48 hours of drug cessation. Factor levels, DIC parameters and reptilase time were normal. The PTT abnormality was consistent with a “lupus like” inhibitor effect (DRVVT was abnormal and the PTT corrected with the addition of phospholipid in two independent tests.) The 2^nd subject treated at DL 4 developed a milder but similar PTT prolongation. The 3^rd subject enrolled on DL 4 had a normal PTT throughout therapy. No subjects developed bleeding complications. Plasma and intracellular drug concentrations were dose related (320–640pg/l and 2–80 ng/5×10e6 cells respectively). Peak drug concentrations were found on Days 3–7. By 14 days after infusion, most ODN was cleared from plasma, but remained measurable in MNC at concentrations 30–50% of the maximum value detected. QRT-PCR for c-myb mRNA was performed in 3 subjects. Subject 1’s (DL 1) c-myb mRNA levels gradually decreased from baseline during infusion and nadired two weeks after cessation of infusion. Subjects 3 (DL 3) & 5 (DL 4) had a decrease in c-myb mRNA levels midway through infusion but c-myb RNA levels increased back to baseline by cessation of infusion. To date no subject has had a clinically important response to therapy. Accrual continues for DL 5.

CONCLUSIONS: C-MYB AS ODN is detectable in plasma and MNCs of subjects during continuous drug infusion with a steady state reached by day 3 of infusion. Plasma drug levels were markedly reduced 14 days after cessation of infusion but MNC drug levels remained elevated. C-MYB AS ODN at DL 4 (10mg/kg/day) is associated with a PTT prolongation consistent with a “lupus inhibitor” like effect. While encouraging biological activity was identified optimal dose and delivery remain to be established before clinically significant effects can be reasonably expected.