Ketoconazole, USP is a white or almost white powder that is soluble in acids.

Ketoconazole is a weak dibasic agent and thus requires acidity for dissolution and absorption.

Mean peak plasma concentrations of approximately 3.5 mcg/mL are reached within 1 to 2 hours, following oral administration of a single 200 mg dose taken with a meal. Oral bioavailability is maximal when the tablets are taken with a meal.

Absorption of ketoconazole tablets is reduced in subjects with reduced gastric acidity, such as subjects taking medications known as acid neutralizing medicines (e.g. aluminum hydroxide) and gastric acid secretion suppressors (e.g. H 2-receptor antagonists, proton pump inhibitors) or subjects with achlorhydria caused by certain diseases (see PRECAUTIONS: Drug Interactions). Absorption of ketoconazole under fasted conditions in these subjects is increased when ketoconazole tablets are administered with an acidic beverage (such as non-diet cola). After pretreatment with omeprazole, a proton pump inhibitor, the bioavailability of a single 200 mg dose of ketoconazole under fasted conditions was decreased to 17% of the bioavailability of ketoconazole administered alone. When ketoconazole was administered with non-diet cola after pretreatment with omeprazole, the bioavailability was 65% of that after administration of ketoconazole alone.

Distribution

In vitro, the plasma protein binding is about 99% mainly to the albumin fraction. Ketoconazole is widely distributed into tissues; however, only a negligible proportion reaches the cerebrospinal fluid.

Metabolism

Following absorption from the gastrointestinal tract, ketoconazole tablets are converted into several inactive metabolites. In vitrostudies have shown that CYP3A4 is the major enzyme involved in the metabolism of ketoconazole. The major identified metabolic pathways are oxidation and degradation of the imidazole and piperazine rings, by hepatic microsomal enzymes. In addition, oxidative O-dealkylation and aromatic hydroxylation does occur. Ketoconazole has not been demonstrated to induce its own metabolism.

Elimination

Elimination from plasma is biphasic with a half-life of 2 hours during the first 10 hours and 8 hours thereafter.

Approximately 13% of the dose is excreted in the urine, of which 2 to 4% is unchanged drug. The major route of excretion is through the bile into the intestinal tract with about 57% being excreted in the feces.

In patients with hepatic or renal impairment, the overall pharmacokinetics of ketoconazole was not significantly different when compared with healthy subjects.

Pediatric Patients

Limited pharmacokinetic data are available on the use of ketoconazole tablets in the pediatric population.

Measurable ketoconazole plasma concentrations have been observed in pre-term infants (single or daily doses of 3 to 10 mg/kg) and in pediatric patients 5 months of age and older (daily doses of 3 to 13 mg/kg) when the drug was administered as a suspension, tablet or crushed tablet. Limited data suggest that absorption may be greater when the drug is administered as a suspension compared to a crushed tablet. Conditions that raise gastric pH may lower or prevent absorption (see PRECAUTIONS: Drug Interactions). Maximum plasma concentrations occurred 1 to 2 hours after dosing and were in the same general range as those seen in adults who received a 200 to 400 mg dose.

Electrocardiogram

Pre-clinical electrophysiological studies have shown that ketoconazole inhibits the rapidly activating component of the cardiac delayed rectifier potassium current, prolongs the action potential duration, and may prolong the QT Cinterval. Data from some clinical PK/PD studies and drug interaction studies suggest that oral dosing with ketoconazole at 200 mg twice daily for 3 to 7 days can result in an increase of the QT Cinterval: a mean maximum increase of about 6 to 12 msec was seen at ketoconazole peak plasma concentrations about 1 to 4 hours after ketoconazole administration.

Ketoconazole blocks the synthesis of ergosterol, a key component of the fungal cell membrane, through the inhibition of cytochrome P-450 dependent enzyme lanosterol 14α-demethylase responsible for the conversion of lanosterol to ergosterol in the fungal cell membrane. This results in an accumulation of methylated sterol precursors and a depletion of ergosterol within the cell membrane thus weakening the structure and function of the fungal cell membrane.