Drug Interactions between Isoptin IV and Technivie

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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ADJUST DOSING INTERVAL: Administration with food may modestly affect the bioavailability of ritonavir from the various available formulations. When the oral solution was given under nonfasting conditions, peak ritonavir concentrations decreased 23% and the extent of absorption decreased 7% relative to fasting conditions. Dilution of the oral solution (within one hour of dosing) with 240 mL of chocolate milk or a nutritional supplement (Advera or Ensure) did not significantly affect the extent and rate of ritonavir absorption. When a single 100 mg dose of the tablet was administered with a high-fat meal (907 kcal; 52% fat, 15% protein, 33% carbohydrates), approximately 20% decreases in mean peak concentration (Cmax) and systemic exposure (AUC) were observed relative to administration after fasting. Similar decreases in Cmax and AUC were reported when the tablet was administered with a moderate-fat meal. In contrast, the extent of absorption of ritonavir from the soft gelatin capsule formulation was 13% higher when administered with a meal (615 KCal; 14.5% fat, 9% protein, and 76% carbohydrate) relative to fasting.

MANAGEMENT: Ritonavir should be taken with meals to enhance gastrointestinal tolerability.

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ADJUST DOSING INTERVAL: Food enhances the oral bioavailability of ombitasvir, paritaprevir, ritonavir, and dasabuvir. Relative to fasting conditions, administration of ombitasvir, paritaprevir, ritonavir, and dasabuvir with a moderate-fat meal (approximately 600 Kcal; 20% to 30% calories from fat) increased the mean systemic exposure (AUC) by 82%, 211%, 49%, and 30%, respectively. Relative to fasting conditions, administration of ombitasvir, paritaprevir, ritonavir, and dasabuvir with a high-fat meal (approximately 900 Kcal; with 60% calories from fat) increased the mean AUC by 76%, 180%, 44%, and 22%, respectively.

MANAGEMENT: Ombitasvir/paritaprevir/ritonavir plus dasabuvir should always be administered with a meal. The fat or calorie content does not matter.

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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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