12 CLINICAL PHARMACOLOGY
12.1 Mechanism of Action
Oxaliplatin undergoes nonenzymatic conversion in physiologic solutions to active derivatives via displacement of the labile oxalate ligand. Several transient reactive species are formed, including monoaquo and diaquo DACH platinum, which covalently bind with macromolecules. Both inter- and intrastrand Pt-DNA crosslinks are formed. Crosslinks are formed between the N7 positions of two adjacent guanines (GG), adjacent adenine-guanines (AG), and guanines separated by an intervening nucleotide (GNG). These crosslinks inhibit DNA replication and transcription. Cytotoxicity is cell-cycle nonspecific.
In vivo studies have shown antitumor activity of oxaliplatin against colon carcinoma. In combination with fluorouracil, oxaliplatin exhibits in vitro and in vivo antiproliferative activity greater than either compound alone in several tumor models (HT29 [colon], GR [mammary], and L1210 [leukemia]).
A pharmacodynamic relationship between platinum ultrafiltrate levels and clinical safety and effectiveness has not been established.
The reactive oxaliplatin derivatives are present as a fraction of the unbound platinum in plasma ultrafiltrate. After a single 2-hour intravenous infusion of Oxaliplatin at a dose of 85 mg/m2, pharmacokinetic parameters expressed as ultrafiltrable platinum were Cmax of 0.814 mcg/mL and volume of distribution of 440 L.
Interpatient and intrapatient variability in ultrafiltrable platinum exposure (AUC0–48hr) assessed over 3 cycles was 23% and 6%, respectively.
At the end of a 2-hour infusion of Oxaliplatin, approximately 15% of the administered platinum is present in the systemic circulation. The remaining 85% is rapidly distributed into tissues or eliminated in the urine. The decline of ultrafiltrable platinum levels following Oxaliplatin administration is triphasic, including two distribution phases (t½α; 0.43 hours and t½β; 16.8 hours).
In patients, plasma protein binding of platinum is irreversible and is greater than 90%. The main binding proteins are albumin and gamma-globulins.
Platinum also binds irreversibly and accumulates (approximately 2-fold) in erythrocytes, where it appears to have no relevant activity. No platinum accumulation was observed in plasma ultrafiltrate following 85 mg/m2 every two weeks.
The decline of ultrafiltrable platinum concentrations from plasma is characterized by a long terminal elimination phase (t½γ; 392 hours).
Oxaliplatin undergoes rapid and extensive nonenzymatic biotransformation. There is no evidence of cytochrome P450-mediated metabolism in vitro.
Up to 17 platinum-containing derivatives have been observed in plasma ultrafiltrate samples from patients, including several cytotoxic species (monochloro DACH platinum, dichloro DACH platinum, and monoaquo and diaquo DACH platinum) and a number of noncytotoxic, conjugated species.
The major route of platinum elimination is renal excretion. At five days after a single 2-hour infusion of Oxaliplatin, urinary elimination accounted for about 54% of the platinum eliminated, with fecal excretion accounting for only about 2%. Platinum was cleared from plasma at a rate (10–17 L/h) that was similar to or exceeded the average human glomerular filtration rate (GFR; 7.5 L/h). The renal clearance of ultrafiltrable platinum is significantly correlated with GFR.
Patients with renal impairment
Patients with normal function (CLcr greater than 80 mL/min) and patients with mild (CLcr=50–80 mL/min) and moderate (CLcr equal to 30–49 mL/min) renal impairment received Oxaliplatin 85 mg/m2 and those with severe (CLcr less than 30 mL/min) renal impairment received Oxaliplatin 65 mg/m2. Mean dose adjusted AUC of unbound platinum was 40%, 95%, and 342% higher for patients with mild, moderate, and severe renal impairment, respectively, compared to patients with normal renal function. Mean dose adjusted Cmax of unbound platinum appeared to be similar among the normal, mild and moderate renal function groups, but was 38% higher in the severe group than in the normal group [see Dosage and Administration (2.3)].
Drug Interaction Studies
No pharmacokinetic interaction between Oxaliplatin 85 mg/m2 and infusional fluorouracil has been observed in patients treated every 2 weeks, but increases of fluorouracil plasma concentrations by approximately 20% have been observed with doses of 130 mg/m2 of Oxaliplatin administered every 3 weeks.
In vitro platinum was not displaced from plasma proteins by the following medications: erythromycin, salicylate, sodium valproate, granisetron, and paclitaxel.
In vitro oxaliplatin does not inhibit human cytochrome P450 isoenzymes.