Thiazides may add to or potentiate the action of other antihypertensive drugs.
The thiazides may decrease arterial responsiveness to norepinephrine. This diminution is not sufficient to preclude effectiveness of the pressor agent for therapeutic use. Thiazides have also been shown to increase the responsiveness to tubocurarine.
Lithium generally should not be given with diuretics because they reduce its renal clearance and add a high risk of lithium toxicity. Refer to the package insert on lithium before use of such concomitant therapy.
Acute renal failure has been reported in a few patients receiving indomethacin and formulations containing triamterene and hydrochlorothiazide. Caution is therefore advised when administering non-steroidal anti-inflammatory agents with triamterene and hydrochlorothiazide.
Potassium-sparing agents should be used very cautiously, if at all, in conjunction with angiotensin-converting enzyme (ACE) inhibitors due to a greatly increased risk of hyperkalemia. Serum potassium should be monitored frequently.
Triamterene and quinidine have similar fluorescence spectra; thus, triamterene and hydrochlorothiazide may interfere with the measurement of quinidine.
Carcinogenesis : Long-term studies with triamterene/hydrochlorothiazide combination have not been conducted.
Triamterene : In studies conducted under the auspices of the National Toxicology Program, groups of rats were fed diets containing 0, 150, 300 or 600 ppm triamterene and groups of mice were fed diets containing 0, 100, 200 or 400 ppm triamterene. Male and female rats exposed to the highest tested concentration received triamterene at about 25 and 30 mg/kg/day, respectively. Male and female mice exposed to the highest tested concentration received triamterene at about 45 and 60 mg/kg/day, respectively.
There was an increased incidence of hepatocellular neoplasia (primarily adenomas) in male and female mice at the highest dosage level. These doses represent 7.5 times and 10 times the MRHD of 300 mg/kg (or 6 mg/kg/day based on a 50 kg patient) for male and female mice, respectively when based on body-weight and 0.7 times and 0.9 times the MRHD when based on body-surface area. Although hepatocellular neoplasia (exclusively adenomas) in the rat study was limited to triamterene-exposed males, incidence was not dose-dependent and there was no statistically significant difference from control incidence at any dose level.
Hydrochlorothiazide : Two-year feeding studies in mice and rats, conducted under the auspices of the National Toxicology Program (NTP), treated mice and rats with doses of hydrochlorothiazide up to 600 and 100 mg/kg/day, respectively. On a body-weight basis, these doses are 600 times (in mice) and 100 times (in rats) the Maximum Recommended Human Dose (MRHD) for the hydrochlorothiazide component of triamterene and hydrochlorothiazide tablets (50 mg/day or 1 mg/kg/day based on a 50 kg patient). On the basis of body-surface area, these doses are 56 times (in mice) and 21 times (in rats) the MRHD. These studies uncovered no evidence of carcinogenic potential of hydrochlorothiazide in rats or female mice, but there was equivocal evidence of hepatocarcinogenicity in male mice.
Studies of the mutagenic potential of the triamterene and hydrochlorothiazide combination have not been performed.
Triamterene : Triamterene was not mutagenic in bacteria (S. typhimurium strains TA 98, TA 100, TA 1535 or TA 1537) with or without metabolic activation. It did not induce chromosomal aberrations in Chinese hamster ovary (CHO) cells in vitro with or without metabolic activation, but it did induce sister chromatid exchanges in CHO cells in vitro with and without metabolic activation.
Hydrochlorothiazide : Hydrochlorothiazide was not genotoxic in in vitro assays using strains TA 98, TA 100, TA 1535, TA 1537 and TA 1538 of Salmonella typhimurium (the Ames test), in the Chinese hamster ovary (CHO) test for chromosomal aberrations, or in in vivo assays using mouse germinal cell chromosomes, Chinese hamster bone marrow chromosomes, and the Drosophila sex-linked recessive lethal trait gene. Positive test results were obtained in the in vitro CHO sister chromatid exchange (clastogenicity) test, and in the mouse lymphoma cell (mutagenicity) assays, using concentrations of hydrochlorothiazide of 43 to 1300 mcg/mL. Positive test results were also obtained in the Aspergillus nidulans nondisjunction assay using an unspecified concentration of hydrochlorothiazide.
Impairment of Fertility : Studies of the effects of the triamterene/ hydrochlorothiazide combination, or of triamterene alone on animal reproductive function have not been conducted.
Hydrochlorothiazide: Hydrochlorothiazide had no adverse effects on the fertility of mice and rats of either sex in studies wherein these species were exposed, via their diet, to doses of up to 100 and 4 mg/kg/day, respectively, prior to mating and throughout gestation. Corresponding multiples of the MRHD are 100 (mice) and 4 (rats) on the basis of body-weight and 9.4 (mice) and 0.8 (rats) on the basis of body-surface area.
Teratogenic Effects: Category C: Animal reproduction studies to determine the potential for fetal harm by triamterene and hydrochlorothiazide tablets have not been conducted. Nevertheless, a One Generation Study in the rat approximated triamterene and hydrochlorothiazide’s composition by using a 1:1 ratio of triamterene to hydrochlorothiazide (30:30 mg/kg/day). There was no evidence of teratogenicity at those doses that were, on a body-weight basis, 15 and 30 times, respectively, the MRHD, and, on the basis of body-surface area, 3.1 and 6.2 times, respectively, the MRHD.
The safe use of triamterene and hydrochlorothiazide tablets in pregnancy has not been established since there are no adequate and well-controlled studies with triamterene and hydrochlorothiazide tablets in pregnant women. Triamterene and hydrochlorothiazide tablets should be used during pregnancy only if the potential benefit justifies the risk to the fetus.
Triamterene: Reproduction studies have been performed in rats at doses as high as 20 times the Maximum Recommended Human Dose (MRHD) on the basis of body-weight, and 6 times the MRHD on the basis of body-surface area without evidence of harm to the fetus due to triamterene.
Because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed.
Hydrochlorothiazide: Hydrochlorothiazide was orally administered to pregnant mice and rats during respective periods of major organogenesis at doses up to 3000 and 1000 mg/kg/day, respectively. At these doses, which are multiples of the MRHD equal to 3000 for mice and 1000 for rats, based on body-weight, and equal to 282 for mice and 206 for rats, based on body-surface area, there was no evidence of harm to the fetus. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed.
Nonteratogenic Effects: Thiazides and triamterene have been shown to cross the placental barrier and appear in cord blood. The use of thiazides and triamterene in pregnant women requires that the anticipated benefits be weighed against possible hazards to the fetus. These hazards include fetal or neonatal jaundice, pancreatitis, thrombocytopenia, and possibly other adverse reactions that have occurred in the adult.
Thiazides and triamterene in combination have not been studied in nursing mothers. Triamterene appears in animal milk and this may occur in humans. Thiazides are excreted in human breast milk. If use of the combination drug product is deemed essential, the patient should stop nursing.
Safety and effectiveness in pediatric patients have not been established.
Side effects observed in association with the use of triamterene and hydrochlorothiazide tablets, other combination products containing triamterene/hydrochlorothiazide, and products containing triamterene or hydrochlorothiazide include the following:
Gastrointestinal: jaundice (intrahepatic cholestatic jaundice), pancreatitis, nausea, appetite disturbance, taste alteration, vomiting, diarrhea, constipation, anorexia, gastric irritation, cramping.
Central Nervous System: drowsiness and fatigue, insomnia, headache, dizziness, dry mouth, depression, anxiety, vertigo, restlessness, paresthesias.
Cardiovascular: tachycardia, shortness of breath and chest pain, orthostatic hypotension (may be aggravated by alcohol, barbiturates or narcotics).
Renal: acute renal failure, acute interstitial nephritis, renal stones composed of triamterene in association with other calculus materials, urine discoloration.
Hematologic: leukopenia, agranulocytosis, thrombocytopenia, aplastic anemia, hemolytic anemia and megaloblastosis.
Ophthalmic: xanthopsia, transient blurred vision.
Hypersensitivity: anaphylaxis, photosensitivity, rash, urticaria, purpura, necrotizing angiitis (vasculitis, cutaneous vasculitis), fever, respiratory distress including pneumonitis.
Other: muscle cramps and weakness, decreased sexual performance and sialadenitis.
Whenever adverse reactions are moderate to severe, therapy should be reduced or withdrawn.
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