Pentostatin

PENTOSTATIN- pentostatin injection, powder, lyophilized, for solution
Mylan Institutional LLC

DESCRIPTION

Pentostatin for injection is supplied as a sterile, apyrogenic, lyophilized powder in single-dose vials for intravenous administration. Each vial contains 10 mg of pentostatin and 50 mg of Mannitol, USP. The pH of the final product is maintained between 7.0 and 8.5 by addition of sodium hydroxide or hydrochloric acid.

Pentostatin, also known as 2’-deoxycoformycin (DCF), is a potent inhibitor of the enzyme adenosine deaminase and is isolated from fermentation cultures of Streptomyces antibioticus. Pentostatin is known chemically as (R)-3-(2-deoxy-ß-D-erythropentofuranosyl)3,6,7,8 tetrahydroimidazo[4,5d][1,3]diazepin-8-ol with a molecular formula of C₁₁ H₁₆ N₄ O₄ and a molecular weight of 268.27. The molecular structure of pentostatin is:

Pentostatin is a white to off-white solid, freely soluble in distilled water.

CLINICAL PHARMACOLOGY

Mechanism of Action

Pentostatin is a potent transition state inhibitor of the enzyme adenosine deaminase (ADA). The greatest activity of ADA is found in cells of the lymphoid system with T-cells having higher activity than B-cells, and T-cell malignancies having higher ADA activity than B-cell malignancies. Pentostatin inhibition of ADA, particularly in the presence of adenosine or deoxyadenosine, leads to cytotoxicity, and this is believed to be due to elevated intracellular levels of dATP which can block DNA synthesis through inhibition of ribonucleotide reductase. Pentostatin can also inhibit RNA synthesis as well as cause increased DNA damage. In addition to elevated dATP, these mechanisms may also contribute to the overall cytotoxic effect of pentostatin. The precise mechanism of pentostatin’s antitumor effect, however, in hairy cell leukemia is not known.

Pharmacokinetics/Drug Metabolism

A tissue distribution and whole-body autoradiography study in the rat revealed that radioactivity concentrations were highest in the kidneys with very little central nervous system penetration.

In man, following a single-dose of 4 mg/m² of pentostatin infused over 5 minutes, the distribution half-life was 11 minutes, the mean terminal half-life was 5.7 hours, the mean plasma clearance was 68 mL/min/m² , and approximately 90% of the dose was excreted in the urine as unchanged pentostatin and/or metabolites as measured by adenosine deaminase inhibitory activity. The plasma protein binding of pentostatin is low, approximately 4%.

A positive correlation was observed between pentostatin clearance and creatinine clearance (CL_cr ) in patients with CL_cr values ranging from 60 mL/min to 130 mL/min.¹ Pentostatin half-life in patients with renal impairment (CL_cr < 50 mL/min, n = 2) was 18 hours, which was much longer than that observed in patients with normal renal function (CL_cr > 60 mL/min, n = 14), about 6 hours.

CLINICAL STUDIES

The following table provides efficacy results for 4 groups (columns) of patients with hairy cell leukemia: patients who initially received pentostatin, patients who initially received alpha-interferon (IFN), and 2 different groups of patients who received pentostatin after proving to be refractory to, or intolerant of IFN therapy. The first 2 groups represent treatment results from the SWOG 8691 study, a large multicenter study comparing pentostatin and IFN in untreated (frontline) patients with confirmed hairy cell leukemia. The third group represents evaluable patients from the SWOG study who crossed over to pentostatin after initially receiving IFN. The fourth group, labeled NCI Phase 2 studies, displays pooled results of 2 noncomparative studies (MD Anderson and CALGB), in which pentostatin was used to treat patients with confirmed IFN-refractory disease.

In the SWOG 8691 study, pentostatin was administered at a dose of 4 mg/m² every 2 weeks. After 6 months of treatment, patients were evaluated for response. If a complete response was achieved, 2 additional doses of pentostatin were administered and then discontinued. If a partial response was achieved, pentostatin was ^continued for up to an additional 6 months. Pentostatin was discontinued for stable disease after 6 months or progressive disease after 2 months of therapy. IFN was administered 3 million units subcutaneously 3 times per week. Patients who achieved a complete or partial response after 6 months of treatment continued on IFN for another 6 months. IFN was discontinued if patients did not achieve a complete or partial response after 6 months of initial treatment or progressed after 2 months. This study allowed crossover of patients intolerant of, or refractory to, initial treatment.

Interferon-refractory patients enrolled into the MD Anderson study received pentostatin at a dose of 4 mg/m² every other week for 3 months and responding patients received 3 additional months. CALGB patients received 4 mg/m² of pentostatin every other week for 3 months and responding patients were treated monthly for up to 9 additional months. Almost all patients had a PS of 0 to 2 in the Phase 2 and 3 studies.

For each study, a complete response (CR) required clearing of the peripheral blood and bone marrow of all hairy cells, normalization of organomegaly and lymphadenopathy by physical examination, and recovery of hemoglobin to at least 12 g/dL, platelet count to at least 100,000/mm³ , and granulocyte count to at least 1500/mm³. A partial response (PR) required that the percentage of hairy cells in the blood and bone marrow decrease by more than 50%, enlarged organs and lymph nodes decrease by more than 50% by physical examination, and hematologic parameters had to meet the same criteria as for complete response. The table below reports the response rate for 2 groups of patients: (1) Evaluable, i.e., patients who could be evaluated for response and (2) Intent-to-Treat, i.e., patients diagnosed with hairy cell leukemia.

NR = Not reached by Kaplan-Meier method; ANC = Absolute neutrophil count.a Evaluable patientsb Patients either refractory to, or intolerant of, IFNc Kaplan-Meier estimate
	FRONTLINE		IFN-REFRACTORYa
Parameter	Evaluable Pentostatin N = 138	Evaluable IFN N = 130	SWOG 8691b Crossover N = 79	NCI Phase 2 Studies N = 44
Response Rates (%)
Evaluable CR	84	18	85	58
PR	6	24	4	28
Intent-to-Treat	N = 170	N = 170
CR	68	14
PR	5	18
Median Time to Response (months)
CR	6.6	11.5	6.0	4.2
PR	4.0	6.2	5.8	—
Median Duration of Response (months)
CR	NR	8.3	NR	> 7.7c (CALGB) >15.2c (MDA)
PR	NR	15.2	NR	—
% Estimated to be in Response After 24 Months
CR	76	16	85	—
PR	50	21	—	—
Median Time to Recovery (days)
ANC (1500/mm3)	70	106	—	—
Platelets (100,000/mm3)	22	36	—	—

The results show that frontline patients treated with pentostatin achieved a significantly higher rate of response than those treated with IFN. The time to recovery of neutrophil and platelet counts was shorter with pentostatin treatment and the estimated duration of response was longer. The response rate in IFN-refractory patients treated with pentostatin was similar to that in pentostatin-treated frontline patients. At a median follow-up duration of 46 months, there was no statistically significant difference in survival between hairy cell leukemia patients initially treated with pentostatin and those initially treated with IFN. However, no definite conclusions regarding survival can be made from these results because they are complicated by the fact that the majority of IFN patients crossed over to pentostatin treatment.

In the Phase 3 SWOG study, 25 patients with hairy cell leukemia died during treatment or follow-up: 18 patients had last received pentostatin (3 of whom had crossed over from IFN), and 7 patients had last received IFN (1 of whom crossed over from pentostatin). Eleven of the 25 deaths occurred within 60 days of the last dose of treatment. Of these, hairy cell leukemia was cited by the investigators as a contributory cause for 1 death in the pentostatin group and 3 deaths in the IFN group. Additionally, infection contributed to the deaths of 3 patients in the pentostatin group and 2 patients in the IFN group. Approximately 4% of hairy cell leukemia patients, in each arm, died more than 60 days after the last dose of either treatment and there was no outstanding cause of death among these patients.