Effect of tetracycline on mefloquine pharmacokinetics in Thai males

Effects of tetracycline on the pharmacokinetics of halofantrine in healthy volunteers

AIMS

To investigate the effect of tetracycline co-administration on the pharmacokinetics of halofantrine in healthy subjects.

METHODS

Eight healthy males were each given 500 mg single oral doses of halofantrine alone, or with tetracycline (500 mg 12 hourly for 7 days), in a crossover fashion. Blood samples collected at predetermined intervals were analyzed for halofantrine and its major metabolite, desbutylhalofantrine (HFM), using a validated HPLC method.

RESULTS

Co-administration of tetracycline and halofantrine resulted in a significant increase (P < 0.05) in the maximum plasma concentration (C(max)), total area under the concentration-time curve (AUC), and terminal elimination half-life (t(1/2,z)), compared with halofantrine alone. (C(max) 0.43 +/- 0.14 vs 1.06 +/- 0.44 microg ml(-1) (95% CI on the difference 0.30, 0.95); AUC 32.0 +/- 13.6 vs 63.7 +/- 20.1 microg ml(-1) h (95% CI 14.2, 49.1); t(1/2,z:) 90.8 +/- 17.9 vs 157.4 +/- 57.4 h (95% CI 21.7, 111.5)). Similarly, tetracycline caused a significant increase (P < 0.05) in the AUC and C(max) of HFM.

CONCLUSIONS

Tetracycline co-administration significantly increases the plasma concentrations of halofantrine and its major metabolite.

Pharmacokinetic interactions of antimalarial agents

Combination of antimalarial agents has been introduced as a response to widespread drug resistance. The higher number of mutations required to express complete resistance against combinations may retard the further development of resistance. Combination of drugs, especially with the artemisinin drugs, may also offer complete and rapid eradication of the parasite load in symptomatic patients and thus reduce the chance of survival of resistant strains. The advantages of combination therapy should be balanced against the increased chance of drug interactions. During the last decade, much of the pharmacokinetics and metabolic pathways of antimalarial drugs have been elucidated, including the role of the cytochrome P450 (CYP) enzyme complex. Change in protein binding is not a significant cause of interactions between antimalarial agents. CYP3A4 and CYP2C19 are frequently involved in the metabolism of antimalarial agents. Quinidine is a potent inhibitor of CYP2D6, but it appears that this enzyme does not mediate the metabolism of any other antimalarial agent. The new combinations proguanil-atovaquone and chlorproguanil-dapsone do not show significant interactions. CYP2B6 and CYP3A4 are involved in the metabolism of artemisinin and derivatives, but further studies may reveal involvement of more enzymes. Artemisinin may induce CYP2C19. Several artemisinin drugs suffer from auto-induction of the first-pass effect, resulting in a decline of bioavailability after repeated doses. The mechanism of this effect is not yet clear, but induction by other agents cannot be excluded. The combination of artemisinin drugs with mefloquine and the fixed combination artemether-lumefantrine have been studied widely, and no significant drug interactions have been found. The artemisinin drugs will be used at an increasing rate, particularly in combination with other agents. Although clinical studies have so far not shown any significant interactions, drug interactions should be given appropriate attention when other combinations are used.

Mefloquine pharmacokinetics in healthy subjects and in peptic ulcer patients after cimetidine administration

The pharmacokinetics of orally administered mefloquine were determined in six healthy male subjects and in six ulcer patients before and after a 3-day course of cimetidine (400 mg morning and evening). Peak plasma concentrations Cmax and AUC0-infinity were similarly and significantly (P < 0.05) increased after cimetidine pretreatement in both healthy subjects and peptic ulcer patients Cmax was increased by 42.4% and 20.5% while AUC0-infinity was increased by 37.5% in healthy and peptic ulcer subjects respectively. The values of t1/2ab absorption and t1/2 beta elimination, total crearance CLT/F and volume of distribution were altered to varying levels after cimetidine treatment but the changes were not statistically significant in both healthy and peptic ulcer subjects. The established long t1/2 beta and this apparent interaction between mefloquine and cimetidine which resulted in increased mefloquine plasma concentration might be of clinical significant in patients with neurological/psychiatric history.

A physicochemical study of the tetracycline coordination to oxovanadium(IV)

The interaction of tetracycline and oxovanadium(IV) in aqueous solution was studied by potentiometric and spectrophotometric methods. Oxovanadium(IV) ions form both a positively charged 1:1 and a neutral 2:1 metal-ligand complex with tetracycline. When a 1:1 ligand-to-metal ratio mixture is used at about pH 4.5 the 1:1 species predominates, being replaced at pH 6 by the binuclear complex. The binuclear complex has been isolated and fully characterised. Infrared and EPR studies suggest the existence of two distinct vanadyl binding sites. Our results indicate that the first vanadium coordinates to the BCD-ring system and the second one to the A-ring. Biological implications of the existence of a neutral complex at physiological pH are briefly discussed.

Management of HIV-infected pregnant patients in malaria-endemic areas: therapeutic and safety considerations in concomitant use of antiretroviral and antimalarial agents

Chemotherapy in pregnancy is an intricate process requiring prudent use of pharmacologic agents. Malarial infection during pregnancy is often fatal, and prophylaxis against the causative parasite necessitates rational therapeutic intervention. Various agents have been used for prophylaxis against malaria during pregnancy, including chloroquine, mefloquine, proguanil, pyrimethamine, and pyrimethamine-sulfadoxine. Use of these agents has been based on a risk-benefit criterion, without appropriate toxicologic or teratologic evaluation. Some of the aforementioned prophylactic agents have been shown to alter glutathione levels and may exacerbate the oxidation-reduction imbalance attendant on HIV infection. HIV-infected patients traveling to or residing in malaria-endemic areas require protection from malarial infection to avoid placing themselves in double jeopardy. Zidovudine (AZT) is recommended for the prevention of vertical transmission of HIV-1 from mother to child. Other agents, such as lamivudine alone or in combination with AZT, nevirapine, or the HIV-1 protease inhibitors, are either being considered or are currently undergoing trials for use in preventing vertical transmission of HIV-1 or managing HIV infection in infants and children. Although the potential for antimalarial agents to cause congenital malformations is low when they are used alone, their ability to cause problems when combined with antiretroviral drugs needs to be evaluated. In developing countries that have high birth rates, a high endemicity of malaria, and alarming rates of new cases of HIV, prophylaxis against both diseases with combination agents during pregnancy is a challenge.

Clinical application of mefloquine pharmacokinetics in the treatment of P falciparum malaria

Malaria remains a major public health problem in large areas of the world. One of the major factors responsible for the resurgence is the emergence of Plasmodium falciparum, resistant to available antimalarials. An antimalarial, mefloquine, has been considered since its introduction as a promising alternative antimalarial drug to overcome the situation of widespread multidrug resistant P falciparum. Pharmacokinetic studies of mefloquine have been investigated in several groups of subjects either as mefloquine alone or as combined regimens. The oral absorption of mefloquine is relatively rapid, reaching peak concentrations within 24 hours. Metabolism takes place in the liver, with carboxymefloquine as a major metabolite. Mefloquine has a large apparent volume of distribution of 200 L and is highly bound (98%) to plasma proteins. The elimination is slow; the terminal half-life is 13 10 to 14 days in Thai patients with falciparum malaria. Vomiting within 1 hour of drug administration has an influence on blood concentrations of mefloquine and this may result in treatment failure. The whole blood concentrations of mefloquine on the first two days of treatment are important determinants of parasitological response. There appear to be no pharmacokinetic interactions between mefloquine and the other two components of Fansimef in patients with uncomplicated falciparum malaria. The advantage of this combination over mefloquine alone in multidrug resistant P falciparum is still debatable. However, recent data seem to support the higher efficacy of Fansimef over mefloquine alone. Concurrent administration of antibiotics, ie ampicillin and tetracycline with mefloquine results in a significant increase in maximum concentration, reduction of the apparent volume of distribution and shortening of the terminal elimination half-life of mefloquine. An antiemetic drug metoclopramide accelerates the absorption of mefloquine and increases the maximum concentration. In contrast, mefloquine concentrations are decreased in the presence of an antimalarial, artesunate. Primaquine has no effect on the pharmacokinetics of mefloquine when given concurrently.

Randomised trial of mefloquine-tetracycline and quinine-tetracycline for acute uncomplicated falciparum malaria

The combination of mefloquine plus tetracycline was compared with quinine plus tetracycline in a randomised therapeutic trial in 102 patients with acute uncomplicated falciparum malaria in Thailand. Quinine plus tetracycline is considered the standard treatment for the highly drug-resistant strains of P. falciparum found in this area. Fifty patients received mefloquine (750 mg given immediately, followed by 500 mg 6 h later) with tetracycline and 52 patients received quinine (600 mg every 8 h for seven days) with tetracycline. Tetracycline was administered to both groups in doses of 250 mg four times daily. All patients were admitted to the hospital for 28 days to exclude re-infection. Ninety-three patients completed the study; nine patients left prior to completion of follow-up for reasons unrelated to their treatment. Cure rates for the two groups were 94% (44/47) for mefloquine plus tetracycline and 98% (45/46) for quinine plus tetracycline. Parasite and fever clearance times were shorter for the group treated with mefloquine but the differences were not statistically significant. Nearly all patients (94%) treated with quinine developed cinchonism compared with only 12% treated with mefloquine; all other symptoms following treatment were similar. Thirteen patients (26%) treated with quinine also developed delayed primary attacks of P. vivax during the follow-up period; none developed in the patients treated with mefloquine. These results support the contention that the combination of mefloquine plus tetracycline is equally effective and less toxic than quinine plus tetracycline for treatment of acute uncomplicated falciparum malaria in areas requiring combination therapy for drug resistance.