Drug Interactions between docetaxel and Isoptin SR

GENERALLY AVOID: Coadministration with inhibitors of CYP450 3A4, such as grapefruit juice, may significantly increase the plasma concentrations of docetaxel, which is a substrate of the isoenzyme. Current data suggest that consumption of large quantities of grapefruit juice inhibit both intestinal and hepatic CYP450 3A4 due to certain compounds present in grapefruit. In a pharmacokinetic study consisting of 7 cancer patients, mean dose-normalized docetaxel systemic exposure (AUC) increased by 2.2-fold and clearance decreased by 49% when intravenous docetaxel was given at a reduced dosage of 10 mg/m2 in combination with the potent CYP450 3A4 inhibitor ketoconazole (200 mg orally once daily for 3 days) compared to docetaxel administered alone at 100 mg/m2. In addition, a case report of a 52-year-old woman with esophageal squamous cell carcinoma receiving a twice weekly chemotherapy regimen including intravenous docetaxel (40 mg/m2) reported that docetaxel AUC increased by 65% compared with the AUC target of 1.96 mg*h/L and clearance decreased by 63%, with a 71% reduction in the patient's neutrophil count. In the absence of other CYP450 3A4 inhibitors, these effects were attributed to daily consumption of 250 mL of grapefruit juice, which the patient had been consuming for at least 3 months. Two weeks after the patient ceased the grapefruit juice, the docetaxel AUC was closer to the target value and the neutrophil count reduction was less than 35%.

MANAGEMENT: The use of docetaxel in combination with grapefruit and grapefruit juice should generally be avoided. If concomitant use is required, a reduced dosage of docetaxel should be considered, particularly if used with large amounts of grapefruit juice, and therapeutic drug monitoring of docetaxel considered per local treatment protocols. Patients should be closely monitored for the development of docetaxel toxicity such as myelosuppression, stomatitis, neurotoxicity (e.g., paraesthesia, dysesthesia, pain), myalgia, asthenia, fluid retention, nausea, vomiting, and diarrhea.

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GENERALLY AVOID: Consumption of large quantities of grapefruit juice may be associated with significantly increased plasma concentrations of oral verapamil. The mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. One study reported no significant effect of a single administration of grapefruit juice on the pharmacokinetics of verapamil in ten hypertensive patients receiving chronic therapy. In another study conducted in nine healthy male volunteers, administration of 120 mg oral verapamil twice daily for 3 days following pretreatment with 200 mL grapefruit juice twice daily for 5 days resulted in a 57% increase in S-verapamil peak plasma concentration (Cmax), a 36% increase in S-verapamil systemic exposure (AUC), a 40% increase in R-verapamil Cmax, and a 28% increase in R-verapamil AUC compared to administration following orange juice. Elimination half-life and renal clearance of both S- and R-verapamil were not affected by grapefruit juice, and there were no significant effects on blood pressure, heart rate, or PR interval. A third study reported a 1.63-fold increase in Cmax and a 1.45-fold increase in AUC of (R,S)-verapamil in 24 young, healthy volunteers given verapamil sustained-release 120 mg twice daily for 7 days with 250 mL grapefruit juice four times daily on days 5 through 7. Two subjects developed PR interval prolongation of more than 350 ms during grapefruit juice coadministration. A high degree of interindividual variability has been observed in these studies. The interaction was also suspected in a case report of a 42-year-old woman who developed complete heart block, hypotension, hypoxic respiratory failure, severe anion gap metabolic acidosis, and hyperglycemia following accidental ingestion of three verapamil sustained-release 120 mg tablets over a span of six hours. The patient's past medical history was remarkable only for migraine headaches, for which she was receiving several medications including verapamil. Prior to admission, the patient had a 2-week history of poorly controlled migraine, and the six hours preceding hospitalization she suffered from worsening headache and palpitations progressing to altered sensorium. An extensive workup revealed elevated verapamil and norverapamil levels more than 4.5 times above the upper therapeutic limits. These levels also far exceeded those reported in the medical literature for patients taking verapamil 120 mg every 6 hours, or 480 mg in a 24-hour period. The patient recovered after receiving ventilator and vasopressor support. Upon questioning, it was discovered that the patient had been drinking large amounts of grapefruit juice (3 to 4 liters total) the week preceding her admission due to nausea. No other sources or contributing factors could be found for the verapamil toxicity.

MANAGEMENT: Patients treated with oral verapamil should avoid the consumption of large amounts of grapefruit or grapefruit juice to prevent any undue fluctuations in serum drug levels. Patients should be advised to seek medical attention if they experience edema or swelling of the lower extremities; sudden, unexplained weight gain; difficulty breathing; chest pain or tightness; or hypotension as indicated by dizziness, fainting, or orthostasis.

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GENERALLY AVOID: Verapamil may increase the blood concentrations and intoxicating effects of ethanol. The exact mechanism of interaction is unknown but may involve verapamil inhibition of ethanol metabolism. In 10 healthy, young volunteers, verapamil (80 mg orally every 8 hours for 6 days) increased the mean peak blood concentration (Cmax) and the 12-hour area under the concentration-time curve (AUC) of ethanol (0.8 g/kg single oral dose) by 17% and 30%, respectively, compared to placebo. Verapamil AUCs were positively correlated to increased ethanol blood AUC values. Subjectively (i.e. each subject's perception of intoxication as measured on a visual analog scale), verapamil also significantly increased the area under the ethanol effect versus time curve but did not change the peak effect or time to peak effect.

MANAGEMENT: Patients treated with verapamil should be counseled to avoid alcohol consumption.

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MONITOR: Calcium-containing products may decrease the effectiveness of calcium channel blockers by saturating calcium channels with calcium. Calcium chloride has been used to manage acute severe verapamil toxicity.

MANAGEMENT: Management consists of monitoring the effectiveness of calcium channel blocker therapy during coadministration with calcium products.

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