Phase I Study of GTI-2040, an Antisense to Ribonucleotide Reductase, in Combination with High-Dose Cytarabine in Patients with Acute Myeloid Leukemia

Abstract

Purpose: Inhibition of ribonucleotide reductase reduces the availability of the endogenous pool of deoxycytidine and may increase cytarabine (AraC) cytotoxicity. We performed a phase I dose escalation trial of AraC combined with GTI-2040, a 20-mer antisense oligonucleotide shown in preclinical studies to decrease levels of the R2 subunit of ribonucleotide reductase, to determine the maximum tolerated dose in adults with relapsed/refractory acute myeloid leukemia. Experimental Design: Twenty-three adults (ages 18-59 years) were enrolled in this dose escalation phase I trial, receiving high-dose AraC twice daily combined with infusional GTI-2040. An ELISA-based assay measured plasma and intracellular concentrations of GTI-2040. R2 protein changes were evaluated by immunoblotting in pretreatment and posttreatment bone marrow samples. Results: The maximum tolerated dose was 5 mg/kg/d GTI-2040 (days 1-6) and 3 g/m 2 /dose AraC every 12 hours for 8 doses. Neurotoxicity was dose limiting. Eight patients (35%) achieved complete remission. Mean bone marrow intracellular concentration of GTI-2040 were higher at 120 hours than at 24 hours from the start of GTI-2040 (P = 0.002), suggesting intracellular drug accumulation over time. Reductions in bone marrow levels of R2 protein (>50%) were observed at 24 and 120 hours. Higher baseline R2 protein expression (P = 0.03) and reductions after 24 hours of GTI-2040 (P = 0.04) were associated with complete remission. Conclusions: GTI-2040 and high-dose AraC were coadministered safely with successful reduction of the intended R2 target and encouraging clinical results. The clinical efficacy of this combination will be tested in an upcoming phase II study.Nucleoside analogues constitute the backbone of several primary and salvage chemotherapy regimens for acute myeloid leukemia (AML; refs. 1, 2). By mimicking endogenous nucleosides, these compounds are incorporated into newly synthesized DNA and induce inhibition and chain termination of newly synthesized nucleic acid, thereby leading to apoptosis (3). Among pyrimidine analogues, cytarabine (AraC) differs from the endogenous cytidine counterpart by the presence of an arabinoside rather than a ribose sugar. This compound undergoes enzymatic phosphorylation into the active metabolite Ara-CTP before being incorporated in newly synthesized DNA. A direct correlation between intracellular levels of Ara-CTP and antileukemic effect has been identified, and different strategies to increase levels of the active metabolite have been investigated (4 -7).Inhibition of the ribonucleotide reductase (RNR) has the potential to increase AraC cytotoxicity. Composed of two subunits, R1 and R2, RNR is required for the reductive conversion of ribonucleotides to deoxynucleotides, a crucial rate-limiting step during DNA synthesis and repair (8, 9). Overexpression of RNR, commonly found in malignant cells, increases the endogenous pool of deoxynucleoside triphosphates, a potential mechanism of chemoresistance to nucleoside analogues competing for DN...

Enzyme Kinetics of GTI-2040, a Phosphorothioate Oligonucleotide Targeting Ribonucleotide Reductase

“…Detailed clinical pharmacokinetics of GTI 2040 in AML patients have been published recently (Klisovic et al, 2008).…”

“…Additionally, clinically GTI-2040 is used as a 6-day infusion, and the total steady-state concentrations are in the range of 0.3 M (Klisovic et al, 2008), further justifying the use of the apparent Michaelis-Menten kinetics to describe the GTI-2040 data for approximation of the HLM enzyme kinetics, which yielded the kinetic parameter as shown in Table 2. Notably, the K m value was determined to be 6.33 M, using the unbound substrate concentrations, and this value is greater than both K d values of GTI-2040 in HLMs.…”

“…Notably, the K m value was determined to be 6.33 M, using the unbound substrate concentrations, and this value is greater than both K d values of GTI-2040 in HLMs. This approximation is especially relevant clinically because the steady-state drug concentrations seen in circulation were well above 0.1 M (Klisovic et al, 2008) and would be adequate to estimate the in vitro intrinsic clearance of GTI-2040 in HLMs, which allows for the in vitro-in vivo correlation.…”

“…GTI-2040 has been shown recently to have promising response and acceptable tolerability in phase I clinical trials as a single agent or in combination with cytarabine for the treatment of advanced solid tumors and acute myeloid leukemia (AML) (Desai et al, 2005;Klisovic et al, 2008). Using a highly specific ion-pair HPLC/tandem mass spectrometry method (Griffey et al, 1997;Gilar and Bouvier, 2000;Dai et al, 2005b), we recently reported the metabolism of GTI-2040 as the sequential nucleotide deletion via the 3Ј exonuclease (Wei et al, 2006).…”

Biochemical Modulation of Aracytidine (Ara-C) Effects by GTI-2040, a Ribonucleotide Reductase Inhibitor, in K562 Human Leukemia Cells

“…1). Based on this rationale, a phase I study of GTI-2040 in combination with Ara-C for the treatment of acute myeloid leukemia (AML) was carried out at this institution (14). This study has demonstrated that GTI-2040 and Ara-C can be safely given to AML patients (10).…”