Telithromycin is a strong inhibitor of the cytochrome P450 3A4 system. Co-administration of KETEK tablets and a drug primarily metabolized by the cytochrome P450 3A4 enzyme system may result in increased plasma concentration of the drug co-administered with telithromycin that could increase or prolong both the therapeutic and adverse effects. Therefore, appropriate dosage adjustments may be necessary for the drug co-administered with telithromycin.

The use of KETEK is contraindicated with cisapride. (See CONTRAINDICATIONS and CLINICAL PHARMACOLOGY, Drug-drug interactions)

The use of KETEK is contraindicated with pimozide. Although there are no studies looking at the interaction between KETEK and pimozide, there is a potential risk of increased pimozide plasma levels by inhibition of CYP 3A4 pathways by KETEK as with macrolides. (See CONTRAINDICATIONS)

In a pharmacokinetic study, simvastatin levels were increased due to CYP 3A4 inhibition by telithromycin. (See CLINICAL PHARMACOLOGY, Drug-drug interactions) Similarly, an interaction may occur with lovastatin or atorvastatin but not with statins which are not metabolized by CYP3A4. 

High levels of HMG-CoA reductase inhibitors increase the risk of myopathy and rhabdomyolysis. Use of simvastatin, lovastatin, or atorvastatin concomitantly with KETEK should be avoided. If KETEK is prescribed, therapy with simvastatin, lovastatin, or atorvastatin should be suspended during the course of treatment. Patients concomitantly treated with statins should be carefully monitored for signs and symptoms of myopathy and rhabdomyolysis.

Colchicine is a substrate for both CYP 3A4 and the efflux transporter, P-glycoprotein (P-gp), and a significant increase in colchicine plasma concentration is anticipated when co-administered with strong CYP 3A4 inhibitors such as telithromycin. (See CONTRAINDICATIONS and WARNINGS, Drug interactions).

Monitoring of digoxin side effects or serum levels should be considered during concomitant administration of digoxin and KETEK. (See CLINICAL PHARMACOLOGY, Drug-drug interactions.)

Hypotension, bradyarrhythmia and loss of consciousness have been observed in patients receiving concomitant treatment with calcium channel blockers that are substrates of CYP 3A4 (e.g., verapamil, amlodipine, diltiazem).

Patients should be monitored with concomitant administration of midazolam and dosage adjustment of midazolam should be considered if necessary. Precaution should be used with other benzodiazepines, which are metabolized by CYP 3A4 and undergo a high first-pass effect (e.g., triazolam). (See CLINICAL PHARMACOLOGY, Drug-drug interactions.)

Concomitant treatment of KETEK with rifampin, a CYP 3A4 inducer, should be avoided. Concomitant administration of other CYP 3A4 inducers such as phenytoin, carbamazepine, or phenobarbital is likely to result in subtherapeutic levels of telithromycin and loss of effect. (See CLINICAL PHARMACOLOGY, Other drug interactions.)

In patients treated with metoprolol for heart failure, the increased exposure to metoprolol, a CYP 2D6 substrate, may be of clinical importance. Therefore, co-administration of KETEK and metoprolol in patients with heart failure should be considered with caution. (See CLINICAL PHARMACOLOGY, Drug-drug interactions.)

Spontaneous post-marketing reports suggest that administration of KETEK and oral anticoagulants concomitantly may potentiate the effects of the oral anticoagulants. Consideration should be given to monitoring prothrombin times/INR while patients are receiving KETEK and oral anticoagulants simultaneously.

No specific drug interaction studies have been performed to evaluate the following potential drug-drug interactions with KETEK. However, these drug interactions have been observed with macrolide products.

Drugs metabolized by the cytochrome P450 system such as carbamazepine, cyclosporine, tacrolimus, sirolimus, hexobarbital, and phenytoin: elevation of serum levels of these drugs may be observed when co-administered with telithromycin. As a result, increases or prolongation of the therapeutic and/or adverse effects of the concomitant drug may be observed.

Ergot alkaloid derivatives (such as ergotamine or dihydroergotamine): acute ergot toxicity characterized by severe peripheral vasospasm and dysesthesia has been reported when macrolide antibiotics were co-administered. Without further data, the co-administration of KETEK and these drugs is not recommended.