Paradoxical deterioration in seizure control due to anticonvulsant-induced hypocalcaemia.

Hepatic cytochrome P450 enzyme induction is associated with certain antiepileptic drugs (AEDs) and may result in hypocalcaemia secondary to vitamin D deficiency. We report a case of a 44-year-old man with a history of epilepsy, who presented with breakthrough seizures after having previously been seizure-free for 11 years. Investigations revealed severe hypocalcaemia with a corrected calcium of 1.7 mmol/L. His phenytoin dose was increased, and he was started on calcium supplementation. He was discharged with a corrected calcium level of 2.05 mmol/L but was readmitted 1 week later with further seizures and a corrected calcium of 1.89 mmol/L. 25-hydroxyvitamin D was low. AED-induced hypocalcaemia was suspected, which had been made paradoxically worse by the increase in phenytoin dose. Alfacalcidol was prescribed and he was switched from phenytoin to levetiracetam with resolution of hypocalcaemia and no further seizures. The authors recommend screening for calcium and vitamin D deficiency in patients on enzyme-inducing AEDs.

CYP3A4 induction by drugs: correlation between a pregnane X receptor reporter gene assay and CYP3A4 expression in human hepatocytes.

Induction of cytochrome P450 3A4 (CYP3A4) is determined typically by employing primary culture of human hepatocytes and measuring CYP3A4 mRNA, protein and microsomal activity. Recently a pregnane X receptor (PXR) reporter gene assay was established to screen CYP3A4 inducers. To evaluate results from the PXR reporter gene assay with those from the aforementioned conventional assays, 14 drugs were evaluated for their ability to induce CYP3A4 and activate PXR. Sandwiched primary cultures of human hepatocytes from six donors were used and CYP3A4 activity was assessed by measuring microsomal testosterone 6beta-hydroxylase activity. Hepatic CYP3A4 mRNA and protein levels were also analyzed using branched DNA technology/Northern blotting and Western blotting, respectively. In general, PXR activation correlated with the induction potential observed in human hepatocyte cultures. Clotrimazole, phenobarbital, rifampin, and sulfinpyrazone highly activated PXR and increased CYP3A4 activity; carbamazepine, dexamethasone, dexamethasone-t-butylacetate, phenytoin, sulfadimidine, and taxol weakly activated PXR and induced CYP3A4 activity, and methotrexate and probenecid showed no marked activation in either system. Ritonavir and troleandomycin showed marked PXR activation but no increase (in the case of troleandomycin) or a significant decrease (in the case of ritonavir) in microsomal CYP3A4 activity. It is concluded that the PXR reporter gene assay is a reliable and complementary method to assess the CYP3A4 induction potential of drugs and other xenobiotics.