Idarubicin Hydrochloride Injection

Mechanism Of Action

Idarubicin hydrochloride is a DNA-intercalating analog of daunorubicin which has an inhibitory effect on nucleic acid synthesis and interacts with the enzyme topoisomerase II. The absence of a methoxy group at position 4 of the anthracycline structure gives the compound a high lipophilicity which results in an increased rate of cellular uptake compared with other anthracyclines.

Pharmacokinetics

Pharmacokinetic studies have been performed in adult leukemia patients with normal renal and hepatic function following intravenous administration of 10 to 12 mg/m² of idarubicin daily for 3 to 4 days as a single agent or combined with cytarabine. The plasma concentrations of idarubicin are best described by a two or three compartment open model. The elimination rate of idarubicin from plasma is slow with an estimated mean terminal half-life of 22 hours (range, 4 to 48 hours) when used as a single agent and 20 hours (range, 7 to 38 hours) when used in combination with cytarabine. The elimination of the primary active metabolite, idarubicinol, is considerably slower than that of the parent drug with an estimated mean terminal half-life that exceeds 45 hours; hence, its plasma levels are sustained for a period greater than 8 days.

The disposition profile shows a rapid distributive phase with a very high volume of distribution presumably reflecting extensive tissue binding. Studies of cellular (nucleated blood and bone marrow cells) drug concentrations in leukemia patients have shown that peak cellular idarubicin concentrations are reached a few minutes after injection. Concentrations of idarubicin and idarubicinol in nucleated blood and bone marrow cells are more than a hundred times the plasma concentrations. Idarubicin disappearance rates in plasma and cells were comparable with a terminal half-life of about 15 hours.  The terminal half-life of idarubicinol in cells was about 72 hours. The extent of drug and metabolite accumulation predicted in leukemia patients for Days 2 and 3 of dosing, based on the mean plasma levels and half-life obtained after the first dose, is 1.7- and 2.3-fold, respectively, and suggests no change in kinetics following a daily × 3 regimen. The percentages of idarubicin and idarubicinol bound to human plasma proteins averaged 97% and 94%, respectively, at concentrations similar to maximum plasma levels obtained in the pharmacokinetic studies. The binding is concentration independent. The plasma clearance is twice the expected hepatic plasma flow indicating extensive extrahepatic metabolism.

The primary active metabolite formed is idarubicinol. As idarubicinol has cytotoxic activity, it presumably contributes to the effects of idarubicin.

The drug is eliminated predominately by biliary and to a lesser extent by renal excretion, mostly in the form of idarubicinol.

Pharmacokinetics In Special Populations

Idarubicin studies in pediatric leukemia patients, at doses of 4.2 to 13.3 mg/m²/day × 3, suggest dose independent kinetics. There is no difference between the half-lives of the drug following daily × 3 or weekly × 3 administration. Cerebrospinal fluid (CSF) levels of idarubicin and idarubicinol were measured in pediatric leukemia patients treated intravenously. Idarubicin was detected in 2 of 21 CSF samples (0.14 and 1.57 ng/mL), while idarubicinol was detected in 20 of these 21 CSF samples obtained 18 to 30 hours after dosing (mean = 0.51 ng/mL; range, 0.22 to 1.05 ng/mL). The clinical relevance of these findings is unknown.

The pharmacokinetics of idarubicin have not been evaluated in leukemia patients with hepatic impairment. It is expected that in patients with moderate or severe hepatic dysfunction, the metabolism of idarubicin may be impaired and lead to higher systemic drug levels. The disposition of idarubicin may be also affected by renal impairment. Therefore, a dose reduction should be considered in patients with hepatic and/or renal impairment (see DOSAGE AND ADMINISTRATION).

Drug-Drug Interactions

No formal drug interaction studies have been performed.

Clinical Studies

Four prospective randomized studies, three U.S. and one Italian, have been conducted to compare the efficacy and safety of idarubicin (IDR) to that of daunorubicin (DNR), each in combination with cytarabine as induction therapy in previously untreated adult patients with acute myeloid leukemia (AML). These data are summarized in the following table and demonstrate significantly greater complete remission rates for the IDR regimen in two of the three U.S. studies and significantly longer overall survival for the IDR regimen in two of the three U.S. studies.

There is no consensus regarding optional regimens to be used for consolidation; however, the following consolidation regimens were used in U.S. controlled trials. Patients received the same anthracycline for consolidation as was used for induction.

Studies 1 and 3 utilized 2 courses of consolidation therapy consisting of idarubicin 12 or 13 mg/m² daily for 2 days, respectively (or DNR 50 or 45 mg/m² daily for 2 days), and cytarabine, either 25 mg/m² by IV bolus followed by 200 mg/m² daily by continuous infusion for 4 days (Study 1), or 100 mg/m² daily for 5 days by continuous infusion (Study 3). A rest period of 4 to 6 weeks is recommended prior to initiation of consolidation and between the courses. Hematologic recovery is mandatory prior to initiation of each consolidation course.

Study 2 utilized 3 consolidation courses, administered at intervals of 21 days or upon hematologic recovery. Each course consisted of idarubicin 15 mg/m² IV for 1 dose (or DNR 50 mg/m² IV for 1 dose), cytarabine 100 mg/m² every 12 hours for 10 doses and 6-thioguanine 100 mg/m² orally for 10 doses. If severe myelosuppression occurred, subsequent courses were given with 25% reduction in the doses of all drugs. In addition, this study included 4 courses of maintenance therapy (2 days of the same anthracycline as was used in induction and 5 days of cytarabine).

Toxicities and duration of aplasia were similar during induction on the 2 arms in the U.S. studies except for an increase in mucositis on the IDR arm in one study. During consolidation, duration of aplasia on the IDR arm was longer in all three studies and mucositis was more frequent in two studies. During consolidation, transfusion requirements were higher on the IDR arm in the two studies in which they were tabulated, and patients on the IDR arm in Study 3 spent more days on IV antibiotics (Study 3 used a higher dose of idarubicin).

The benefit of consolidation and maintenance therapy in prolonging the duration of remission and survival is not proven.

Intensive maintenance with idarubicin is not recommended in view of the considerable toxicity (including deaths in remission) experienced by patients during the maintenance phase of Study 2.

A higher induction death rate was noted in patients on the IDR arm in the Italian trial. Since this was not noted in patients of similar age in the U.S. trials, one may speculate that it was due to a difference in the level of supportive care.