Chapter 2The Pre-Travel Consultation
Interactions among Travel Vaccines & Drugs
Vaccines and medications are prescribed frequently in pre-travel consultations, and potential interactions between vaccines and medications, including those already taken by the traveler, must be considered. Although a comprehensive list of interactions is beyond the scope of this section, some of the more serious interactions of commonly used travel-related vaccines and medications are discussed here.
INTERACTIONS BETWEEN VACCINES
In general, concomitant administration of multiple vaccines, including live attenuated immunizations, is safe and effective. Administering a live-virus vaccine within 4 weeks after administration of another live-virus vaccine can decrease immunogenicity to the second administered vaccine. This observation has given rise to the recommendation that live-virus vaccines should be administered the same day or ≥4 weeks apart. If the 4-week span is not achievable, the second vaccine may be administered sooner to afford some protection, but it should be readministered ≥4 weeks later if the traveler is at continued risk. A recent study examining concurrent administration of the yellow fever vaccine with the measles-mumps-rubella (MMR) vaccine in 12-month-old children showed slightly reduced immunogenicity to yellow fever and mumps components, compared with responses following separate vaccination with MMR and yellow fever vaccine 30 days apart. The clinical significance of this finding is uncertain at present.
INTERACTIONS BETWEEN TRAVEL VACCINES AND DRUGS
Oral Typhoid Vaccine
Live attenuated vaccines should generally be avoided in immunocompromised travelers, including those who are taking immunomodulators, calcineurin inhibitors, cytotoxic agents, antimetabolites, and high-dose steroids.
Antimicrobial agents may be active against the vaccine strain in the oral typhoid vaccine and may prevent an adequate immune response to the vaccine. Therefore, oral typhoid vaccine should not be given to people taking antibacterial agents. Vaccination with oral typhoid vaccine should be delayed for >72 hours after the administration of any such agent. Parenteral typhoid vaccine may be a more appropriate choice for these people.
Chloroquine and atovaquone-proguanil at doses used for malaria chemoprophylaxis may be given concurrently with the oral typhoid vaccine.
Concomitant use of chloroquine may reduce the antibody response to intradermal rabies vaccine administered for preexposure vaccination. The intramuscular route should be used for people taking chloroquine concurrently. (Currently, intradermal administration of rabies vaccine is not approved in the United States.)
INTERACTIONS BETWEEN ANTIMALARIALS AND SELECTED OTHER DRUGS
This section describes some of the more commonly encountered drug interactions. Any time a new medication is prescribed, clinicians should check for any interactions and inform the traveler of the potential risk.
Mefloquine may interact with several categories of drugs, including other antimalarial drugs, drugs that alter cardiac conduction, and anticonvulsants. Mefloquine is associated with increased toxicities of the antimalarial drug lumefantrine (available in the United States in fixed combination to treat people with uncomplicated Plasmodium falciparum malaria), potentially causing fatal prolongation of the QTc interval. Lumefantrine should therefore be avoided or used with caution in patients taking mefloquine prophylaxis. Although no conclusive data are available with regard to coadministration of mefloquine and other drugs that may affect cardiac conduction, mefloquine should be used with caution or avoided in patients taking antiarrhythmic or β-blocking agents, calcium-channel blockers, antihistamines, H1-blocking agents, tricyclic antidepressants, or phenothiazines. In general, mefloquine should also be avoided in travelers with a history of seizures. Mefloquine may also lower plasma levels of a number of anticonvulsants, such as valproic acid, carbamazepine, phenobarbital, and phenytoin; concurrent use of mefloquine with these agents should be avoided. Mefloquine can also lead to increased levels of calcineurin inhibitors and mTOR inhibitors (tacrolimus, cyclosporine A, and sirolimus) (see Table 8-02). Potent CYP3A4 inhibitors such as macrolides (clarithromycin, erythromycin), azole antifungals (ketoconazole, voriconazole, and itraconazole), and antiretroviral protease inhibitors may increase the levels of mefloquine, increasing the risk for QT prolongation. CYP3A4 inducers such as efavirenz, nevirapine, rifampin, and rifabutin may reduce plasma concentrations of mefloquine, and concurrent use should be avoided.
Chloroquine may increase risk of prolonged QTc interval when given with other QT-prolonging agents (such as sotalol, amiodarone, and lumefantrine), and the combination should be avoided. Chloroquine inhibits CYP2D6; when given concomitantly with substrates of this enzyme (such as metoprolol, propranolol, fluoxetine, paroxetine, flecainide), increased monitoring for side effects may be warranted. Chloroquine absorption may be reduced by antacids or kaolin; ≥4 hours should elapse between doses of these medications. Concomitant use of cimetidine and chloroquine should be avoided, as cimetidine can inhibit the metabolism of chloroquine and may increase drug levels. CYP3A4 inhibitors such as ritonavir, ketoconazole, and erythromycin may also increase chloroquine levels, and concomitant use should be avoided. Chloroquine inhibits bioavailability of ampicillin, and 2 hours should elapse between doses. Chloroquine is also reported to decrease the bioavailability of ciprofloxacin and methotrexate. Chloroquine may increase digoxin levels; increased digoxin monitoring is warranted. Use of chloroquine could possibly also lead to increased levels of calcineurin inhibitors and should be used with caution.
Tetracycline, rifampin, and rifabutin may reduce plasma concentrations of atovaquone and should not be used concurrently with atovaquone-proguanil. Metoclopramide may reduce bioavailability of atovaquone; unless no other antiemetics are available, this antiemetic should not be used to treat the vomiting that may accompany use of atovaquone at treatment doses. Atovaquone-proguanil should not be used with other medications that contain proguanil. Patients on anticoagulants may need to reduce their anticoagulant dose or monitor their prothrombin time more closely while taking atovaquone-proguanil, although coadministration of these drugs is not contraindicated. Atovaquone-proguanil may interact with antiretroviral protease inhibitors and efavirenz. Cimetidine and fluvoxamine interfere with the metabolism of proguanil and should therefore be avoided.
Phenytoin, carbamazepine, and barbiturates may decrease the half-life of doxycycline. Patients on anticoagulants may need to reduce their anticoagulant dose while taking doxycycline because of its ability to depress plasma prothrombin activity. Absorption of tetracyclines may be impaired by bismuth subsalicylate, preparations containing iron, and antacids containing calcium, magnesium, or aluminum; these preparations should not be taken within 3 hours of taking doxycycline. Doxycycline may interfere with the bactericidal activity of penicillin, so these drugs, in general, should not be taken together. Doxycycline has no known interaction with antiretroviral agents, but concurrent use may lead to increased levels of calcineurin inhibitors and mTOR inhibitors (sirolimus).
INTERACTIONS WITH ANTIDIARRHEAL DRUGS
Increase in the international normalized ratio has been reported when levofloxacin and warfarin are used concurrently. Concurrent administration of ciprofloxacin and antacids that contain magnesium or aluminum hydroxide may reduce bioavailability of ciprofloxacin. Ciprofloxacin decreases clearance of theophylline and caffeine; theophylline levels should be monitored when ciprofloxacin is used concurrently. Ciprofloxacin and other fluoroquinolones should not be used with tizanidine. Sildenafil should not be used in patients on ciprofloxacin, as concomitant use is associated with increased rates of adverse effects. Fluoroquinolones have no known interaction with antiretroviral agents, but concurrent use may increase levels of calcineurin inhibitors and fluoroquinolone levels, and use should reflect renal function.
Close monitoring for side effects of azithromycin is recommended when azithromycin is used with nelfinavir. Increased anticoagulant effects have been noted when azithromycin is used with warfarin; monitoring prothrombin time is recommended for people taking these drugs concomitantly. Additive QTc prolongation may occur when azithromycin is used with the antimalarial artemether, and concomitant therapy should be avoided. Drug interactions have been reported with macrolides and antiretroviral protease inhibitors, as well as efavirenz and nevirapine, and can increase risk of QTc prolongation. Concurrent use with macrolides may lead to increased levels of calcineurin inhibitors.
No clinically significant drug interactions have been reported to date with rifaximin. Although the drug induces CYP3A4 enzymes, studies of concurrent administration of rifaximin with midazolam, and with a single dose of the oral contraceptive ethinylestradiol and norgestimate, did not show changes in the pharmacokinetics of these drugs.
INTERACTIONS WITH DRUGS USED FOR TRAVEL TO HIGH ALTITUDES
Acetazolamide produces alkaline urine that can increase the rate of excretion of barbiturates and salicylates and may potentiate salicylate toxicity, particularly if taking a high dose of aspirin. Decreased excretion of dextroamphetamine, anticholinergics, mecamylamine, ephedrine, mexiletine, or quinidine may also occur. Hypokalemia caused by corticosteroids may be potentiated by concurrent use of acetazolamide. Acetazolamide should not be given to patients taking the anticonvulsant topiramate, as concurrent use is associated with increased toxicity. Increased monitoring of cyclosporine, tacrolimus, and sirolimus is warranted if these drugs are given with acetazolamide. Concurrent administration of metformin and acetazolamide should be done with caution as there may be an additive risk for lactic acidosis.
Dexamethasone interacts with multiple classes of drugs. Using this drug to treat altitude illness may, however, be life-saving. Interactions may occur with dexamethasone and the following drugs and drug classes: macrolide antibiotics, anticholinesterases, anticoagulants, hypoglycemic agents, isoniazid, digitalis preparations, oral contraceptives, and phenytoin.
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