Mechanism of Action
Although the exact mechanism of action of bleomycin is unknown, available evidence indicates that the main mode of action is the inhibition of DNA synthesis with some evidence of lesser inhibition of RNA and protein synthesis. Bleomycin is known to cause single, and to a lesser extent, double-stranded breaks in DNA. In in vitro and in vivo experiments, bleomycin has been shown to cause cell cycle arrest in G2 and in mitosis.
When administered into the pleural cavity in the treatment of malignant pleural effusion, Bleomycin acts as a sclerosing agent.
Bleomycin is rapidly absorbed following either intramuscular, subcutaneous, intraperitoneal, or intrapleural administration reaching peak plasma concentrations in 30 to 60 minutes. Systemic bioavailability of bleomycin is 100% and 70% following intramuscular and subcutaneous administrations, respectively, and 45% following both intraperitoneal and intrapleural administrations, compared to intravenous and bolus administration.
Following intramuscular doses of 1 to 10 units/m2, both peak plasma concentration and AUC increased in proportion with the increase of dose.
Following intravenous bolus administration of 30 units of bleomycin to one patient with a primary germ cell tumor of the brain, a peak CSF level was 40% of the simultaneously-obtained plasma level and was attained in two hours after drug administration. The area under the bleomycin CSF concentration x time curve was 25% of the area of the bleomycin plasma concentration x time curve.
Bleomycin is widely distributed throughout the body with a mean volume of distribution of 17.5 L/m2 in patients following a 15 units/m2 intravenous bolus dose. Protein binding of bleomycin has not been studied.
Bleomycin is inactivated by a cytosolic cysteine proteinase enzyme, bleomycin hydrolase. The enzyme is widely distributed in normal tissues with the exception of the skin and lungs, both targets of bleomycin toxicity. Systemic elimination of the drug by enzymatic degradation is probably only important in patients with severely compromised renal function.
The primary route of elimination is via the kidneys. About 65% of the administered intravenous dose is excreted in urine within 24 hours. In patients with normal renal function, plasma concentrations of bleomycin decline biexponentially with a mean terminal half-life of 2 hours following intravenous bolus administration. Total body clearance and renal clearance averaged 51 mL/min/m2 and 23 mL/min/m2, respectively.
Following intrapleural administration to patients with normal renal function, a lower percentage of drug (40%) is recovered in the urine, as compared to that found in the urine after intravenous administration.
Age, Gender, and Race
The effects of age, gender, and race on the pharmacokinetics of bleomycin have not been evaluated.
Children of less than 3 years of age have higher total body clearance than in adults, 71 mL/min/m2 versus 51 mL/min/m2, respectively, following intravenous bolus administration. Children of more than 8 years of age have comparable clearance as in adults.
In children with normal renal function, plasma concentrations of bleomycin decline biexponentially as in adults. The volume of distribution and terminal half-life of bleomycin in children appears comparable to that in adults.
Renal insufficiency markedly alters bleomycin elimination. The terminal elimination half-life increases exponentially as the creatinine clearance decreases. Dosing reductions were proposed for patients with creatinine clearance values of <50 mL/min (see PRECAUTIONS and DOSAGE AND ADMINISTRATION).
The effect of hepatic insufficiency on the pharmacokinetics of bleomycin has not been evaluated.
Drugs that Can Affect Renal Clearance
Because bleomycin is eliminated predominantly through renal excretion, the administration of nephrotoxic drugs with bleomycin may affect its renal clearance. Specifically, in one report of 2 children receiving concomitant cisplatin with bleomycin, total body clearance of bleomycin decreased from 39 to 18 mL/min/m2 as the cumulative dose of cisplatin exceeded 300 mg/m2. Terminal half-life of bleomycin also increased from 4.4 to 6.0 hours. Fatal bleomycin pulmonary toxicity has been reported in a patient with unrecognized cisplatin-induced oliguric renal failure.
Malignant Pleural Effusion
The safety and efficacy of bleomycin 60 units and tetracycline (1 g) as treatment for malignant pleural effusion were evaluated in a multicenter, randomized trial. Patients were required to have cytologically positive pleural effusion, good performance status (0,1,2), lung re-expansion following tube thoracostomy with drainage rates of 100 mL/24 hours or less, no prior intrapleural therapy, no prior systemic bleomycin therapy, no chest irradiation, and no recent change in systemic therapy. Overall survival did not differ between the bleomycin (n=44) and tetracycline treatment (n=41) groups. Of patients evaluated within 30 days of instillation, the recurrence rate was 36% (10/28) with bleomycin and 67% (18/27) with tetracycline (p=0.023). Toxicity was similar between groups.