Totect (Dexrazoxane Hydrochloride) - Clinical Pharmacology

CLINICAL PHARMACOLOGY

Mechanism of Action

The mechanism by which Totect™ diminishes tissue damage resulting from the extravasation of anthracycline drugs is unknown. Some evidence suggests that dexrazoxane inhibits topoisomerase II reversibly.

Pharmacokinetics/Pharmacodynamics

The pharmacokinetics of dexrazoxane following dosing of patients with anthracycline extravasation have not been studied.

The pharmacokinetics of dexrazoxane have been studied in advanced cancer patients with normal renal and hepatic function. Generally, the pharmacokinetics of dexrazoxane can be adequately described by a two-compartment open model with first-order elimination. Dexrazoxane has been administered as a 15 minute infusion over a dose-range of 60 to 900 mg/m² with 60 mg/m² of doxorubicin, and at a fixed dose of 500 mg/m² with 50 mg/m² doxorubicin. The disposition kinetics of dexrazoxane are dose-independent, as shown by linear relationship between the area under plasma concentration-time curves and administered doses ranging from 60 to 900 mg/m². The mean peak plasma concentration of dexrazoxane was 36.5 μg/mL at the end of the 15 minute infusion of a 500 mg/m² dose of dexrazoxane administered 15 to 30 minutes prior to the 50 mg/m² doxorubicin dose. The important pharmacokinetic parameters of dexrazoxane are summarized in the following table.

SUMMARY OF MEAN (%CV^a) DEXRAZOXANE PHARMACOKINETIC PARAMETERS AT A DOSAGE RATIO OF 10:1 OF DEXRAZOXANE: DOXORUBICIN

a Coefficient of variation
b Steady-state volume of distribution
Dose	Dose	Number of	Elimination	Plasma	Renal	bVolume of
Doxorubicin	Dexrazoxane	Subjects	Half-Life	Clearance	Clearance	Distribution
(mg/m2)	(mg/m2)		(h)	(L/h/m2)	(L/h/m2)	(L/m2)
50	500	10	2.5 (16)	7.88 (18)	3.35 (36)	22.4 (22)
60	600	5	2.1 (29)	6.25 (31)	—	22.0 (55)

Following a rapid distributive phase (~0.2 to 0.3 hours), dexrazoxane reaches post-distributive equilibrium within 2 to 4 hours. The estimated steady-state volume of distribution of dexrazoxane suggests its distribution primarily in the total body water (25 L/m²). The mean systemic clearance and steady-state volume of distribution of dexrazoxane in two Asian female patients at 500 mg/m² dexrazoxane along with 50 mg/m² doxorubicin were 15.15 L/h/m² and 36.27 L/m², respectively, but their elimination half-life and renal clearance of dexrazoxane were similar to those of the ten Caucasian patients from the same study. Qualitative metabolism studies with dexrazoxane have confirmed the presence of unchanged drug, a diacid-diamide cleavage product, and two monoacid-monoamide ring products in the urine of animals and man. The metabolite levels were not measured in the pharmacokinetic studies.

Urinary excretion plays an important role in the elimination of dexrazoxane. Forty-two percent of the 500 mg/m² dose of dexrazoxane was excreted in the urine.

Protein Binding: In vitro studies have shown that dexrazoxane is not bound to plasma proteins.

Special Populations:

Pediatric: The pharmacokinetics of dexrazoxane have not been evaluated in pediatric patients.

Gender: There are no clinically relevant differences in the pharmacokinetics of dexrazoxane between males and females.

Renal insufficiency: The pharmacokinetics of dexrazoxane were assessed following a single 15 minute IV infusion of 150 mg/m² of dexrazoxane in male and female subjects with varying degrees of renal dysfunction as determined by creatinine clearance (CL_CR) based on a 24-hour urinary creatinine collection. Dexrazoxane clearance was reduced in subjects with renal dysfunction. Compared with controls, the mean AUC_0-inf value was twofold greater in subjects with moderate (CL_CR 30-50 mL/min) to severe (CL_CR <30 mL/min) renal dysfunction. Modeling demonstrated that equivalent exposure (AUC_0-inf) could be achieved if dosing were reduced by 50% in subjects with creatinine clearance values < 40 mL/min compared with control subjects (CL_CR >80 mL/min) (see PRECAUTIONS, DOSAGE AND ADMINISTRATION).

Hepatic insufficiency: The pharmacokinetics of dexrazoxane have not been evaluated in patients with hepatic impairment.

Drug Interactions: There were no significant changes in the pharmacokinetics of doxorubicin (50 mg/m²) and its predominant metabolite, doxorubicinol, in the presence of dexrazoxane (500 mg/m²) in a crossover study in cancer patients.

CLINICAL STUDIES

Totect™ was studied in two open-label, single arm, multi-center studies testing whether Totect™ administration could reduce tissue injury following anthracycline extravasation and thereby reduce or avoid surgical intervention.

In the two studies, eligible patients were receiving single-agent anthracycline intravenously (usually as part of combination chemotherapy) and developed extravasation symptoms of pain, burning, swelling, and/or redness near the infusion site. Skin biopsy samples from the suspected area were examined for the presence of anthracycline as determined by the presence of tissue fluorescence; however, therapy was not delayed for this test result.

In both studies, treatment with Totect™ was to begin as soon as possible and no later than 6 hours after extravasation with retreatment 24 and 48 hours later (a total of 3 doses). Totect™ was administered as 1-2 hour IV infusions through a different venous access location. The first and second doses were 1000 mg/m² and the third dose was 500 mg/m². No dose modifications were planned except for patients whose body surface area exceeded 2.0 m², in which case the total daily dose limit on the first and second day was 2000 mg/day and 1000 mg on the third day.

In total, 80 patients were enrolled and 57 were evaluable. Demographics in the two studies were similar. The median age was 57 years, and sixty-five percent of patients were women. The anthracyclines most commonly associated with extravasation were epirubicin (56%) and doxorubicin (41%). Peripheral IV sites of extravasation included the forearm in 63%, the hand in 21%, and the antecubital area in 11%; four patients (5%) received the anthracycline via a central venous access device (CVAD). Most patients presented with swelling (83%), redness (78%), and pain (43%). In study 1, 11% also presented with blisters. The median baseline lesion area was 25 cm² (range 1-253 cm²).

Evaluable patients had to be receiving single-agent IV anthracycline at the time of extravasation, to have skin biopsies showing fluorescence, and to receive the first Totect™ dose within 6 hours of the extravasation.

In study 1, none of the 19 evaluable patients required surgical intervention and none had serious late sequelae. In study 2, one of the 38 evaluable patients required surgery. One additional non-evaluable patient required surgery for tissue necrosis. Thirteen patients had late sequelae at the event site such as site pain, fibrosis, atrophy, and local sensory disturbance; all were judged as mild except in the one patient who required surgery. None of the 4 patients with CVADs required surgical intervention.