Hydrochlorothiazide: Hydrochlorothiazide is not metabolized by CYP enzymes and does not induce or inhibit these enzymes. It is therefore regarded as probably not porphyrinogenic. Telmisartan: Telmisartan is not metabolized by CYP450 enzymes. Telmisartan has been found to be an inhibitor of CYP2C19 and CYP2C9 in vitro, but this has not been observed in vivo.

Hydrochlorothiazide: Sulfonamide congener. Telmisartan: Angiotensin II receptor antagonist

Hydrochlorothiazide: Hydrochlorothiazide is a thiazide diuretic that inhibits reabsorption of electrolytes in the distal tubule, leading to increased excretion of water, sodium, chloride, potassium, magnesium and other electrolytes. It is administered orally. Telmisartan: Telmisartan is used in the treatment of essential hypertension. It is administered orally.

Telmisartan: Oral administration of 40 mg telmisartan gave a Cmax of 44.7 ng/ml (Wienen 2000), which is equivalent to 87 nM.

Hydrochlorothiazide: Hydrocholorothiazide is not metabolized, but is eliminated mainly unchanged in the urine. The plasma half-life is about 9,4 – 13 hours (Beermann 1976, Drugbank, SPC). Telmisartan: Telmisartan is metabolized by glucuronidation and no CYP450 enzymes are thought to be involved (SPC). Telmisartan was found to be an in vitro inhibitor of CYP2C9, however such inhibition is unlikely to be clinically relevant due to the low free plasma concentration obtained at therapeutic doses (Kamiyama 2007). An in vivo study with co-administration of telmisartan and the CYP2C9 substrate, warfarin, did not demonstrate a clinically relevant drug-drug interaction (Stangier 2000). In vitro testing showed that telmisartan inhibited CYP2C19, but this was not clinically relevant (Wienen 2000).

Hydrochlorothiazide: In an in ovo study of chick embryo livers, methyclothiazide, another thiazide drug, was found to have potential for induction of ALAS, whereas hydroclorothiazide and other thiazides was found not to have these effects (Anderson 1981). The results from these types of animal studies are not directly transferable to human, and cannot be given weight in the judgement of porphyrinogeniciry of drugs (Hift 2011).

Hydrochlorothiazide: Thunell, patient report (n=1): tolerated. Andersson, patient reports (n=10): tolerated. Peters: On basis of clinical experience probably not porphyrinogenic.

Hydrochlorothiazide: Uneventful use reported in 9 patients with acute porphyria. Telmisartan: Uneventful use reported in 1 patient with acute porphyria.