Drug Interactions between AccessPak for HIV PEP Expanded with Kaletra and Versacloz

MONITOR CLOSELY: Clozapine has the potential to prolong QT interval of the electrocardiogram. Theoretically, coadministration with other agents that can cause QT prolongation may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. Clozapine treatment alone has been associated with ventricular arrhythmia, torsade de pointes, cardiac arrest, and sudden death. In general, the risk of an individual agent or a combination of agents causing ventricular arrhythmia in association with QT prolongation is largely unpredictable but may be increased by certain underlying risk factors such as congenital long QT syndrome, cardiac disease, and electrolyte disturbances (e.g., hypokalemia, hypomagnesemia, hypocalcemia). In addition, the extent of drug-induced QT prolongation is dependent on the particular drug(s) involved and dosage(s) of the drug(s). Certain QT-prolonging agents (e.g., neuroleptics; phenothiazines; tricyclic antidepressants; some antihistamines, antispasmodics, and skeletal muscle relaxants) may also have additive parasympatholytic and central nervous system-depressant effects with clozapine. Excessive parasympatholytic effects can result in paralytic ileus, hyperthermia, mydriasis, blurred vision, tachycardia, urinary retention, psychosis, and seizures.

MANAGEMENT: Caution is recommended if clozapine is used in combination with other drugs that can prolong the QT interval. Serum electrolytes, including potassium, magnesium and calcium, should be measured at baseline and periodically during treatment, and any abnormalities corrected prior to initiating clozapine. Routine ECG assessment may detect QTc prolongation, but is not always effective in preventing arrhythmias. Clozapine treatment should be discontinued if the QTc interval exceeds 500 msec. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope. Additional precaution is required when using QT-prolonging agents with anticholinergic properties, particularly in the elderly and those with underlying organic brain disease. Patients should be advised to notify their physician if they experience potential symptoms of anticholinergic intoxication such as abdominal pain, fever, heat intolerance, blurred vision, confusion, and/or hallucinations. Dosage adjustments may be necessary if excessive adverse effects develop. Ambulatory patients should be counseled to avoid activities requiring mental alertness until they know how these agents affect them.

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MONITOR: Coadministration with ritonavir may theoretically increase the serum concentrations of clozapine. The proposed mechanism is ritonavir inhibition of clozapine metabolism via CYP450 3A4 and 2D6. However, no pharmacokinetic data are currently available. Clozapine is also metabolized by CYP450 1A2. Because it is a substrate for several different CYP450 enzymes, the risk of metabolic interactions caused by an effect on an individual isoform is thought to be minimized. Nevertheless, the manufacturer recommends caution during concomitant treatment of clozapine with other drugs that are inhibitors of these enzymes.

MANAGEMENT: Caution is advised if clozapine is prescribed in combination with ritonavir. Pharmacologic response to clozapine should be monitored more closely whenever ritonavir is added to or withdrawn from therapy in patients stabilized on their existing antipsychotic regimen, and the clozapine dosage adjusted as necessary.

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MONITOR: Coadministration with ritonavir, with or without lopinavir, has been suggested in postmarketing reports to increase the proximal tubular intracellular concentrations of tenofovir and potentiate the risk of tenofovir-induced nephrotoxicity. The proposed mechanism is ritonavir inhibition of tenofovir renal tubular secretion into the urine via multidrug resistance protein MRP2. Analysis of data from a compassionate access study in which 271 patients with advanced HIV disease received the combination for a mean duration of 63 weeks revealed no clinically significant nephrotoxicity associated with coadministration. However, there have been case reports of renal failure associated with acute tubular necrosis, Fanconi's syndrome, and nephrogenic diabetes insipidus in patients treated with tenofovir disoproxil fumarate in combination with ritonavir. Some patients had incomplete recovery of renal function more than a year after cessation of tenofovir therapy. Ritonavir given in combination with lopinavir has also been reported to modestly increase the plasma concentrations of tenofovir. In contrast, both slight decreases and no change in lopinavir and ritonavir concentrations have been reported.

MANAGEMENT: Caution is advised if tenofovir disoproxil fumarate is prescribed with ritonavir. Renal function should be monitored regularly, including surveillance for signs of tubulopathy such as glycosuria, acidosis, increases in serum creatinine level, electrolyte disturbances (e.g., hypokalemia, hypophosphatemia), and proteinuria. The same precaution may be applicable during therapy with other protease inhibitors based on their similar pharmacokinetic profile, although clinical data are lacking. Nelfinavir reportedly does not alter the pharmacokinetics of tenofovir, or vice versa. Tenofovir administration should be discontinued promptly if nephropathy develops.

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MONITOR: Coadministration with ritonavir, with or without lopinavir, has been suggested in postmarketing reports to increase the proximal tubular intracellular concentrations of tenofovir and potentiate the risk of tenofovir-induced nephrotoxicity. The proposed mechanism is ritonavir inhibition of tenofovir renal tubular secretion into the urine via multidrug resistance protein MRP2. Analysis of data from a compassionate access study in which 271 patients with advanced HIV disease received the combination for a mean duration of 63 weeks revealed no clinically significant nephrotoxicity associated with coadministration. However, there have been case reports of renal failure associated with acute tubular necrosis, Fanconi's syndrome, and nephrogenic diabetes insipidus in patients treated with tenofovir disoproxil fumarate in combination with ritonavir. Some patients had incomplete recovery of renal function more than a year after cessation of tenofovir therapy. Ritonavir given in combination with lopinavir has also been reported to modestly increase the plasma concentrations of tenofovir. In contrast, both slight decreases and no change in lopinavir and ritonavir concentrations have been reported.

MANAGEMENT: Caution is advised if tenofovir disoproxil fumarate is prescribed with ritonavir. Renal function should be monitored regularly, including surveillance for signs of tubulopathy such as glycosuria, acidosis, increases in serum creatinine level, electrolyte disturbances (e.g., hypokalemia, hypophosphatemia), and proteinuria. The same precaution may be applicable during therapy with other protease inhibitors based on their similar pharmacokinetic profile, although clinical data are lacking. Nelfinavir reportedly does not alter the pharmacokinetics of tenofovir, or vice versa. Tenofovir administration should be discontinued promptly if nephropathy develops.

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