Chemical formula: C₂₂H₂₂F₃N Molecular mass: 357.412 g/mol PubChem compound: 156419
The calcium sensing receptor on the surface of the chief cell of the parathyroid gland is the principal regulator of PTH secretion. Cinacalcet is a calcimimetic agent which directly lowers PTH levels by increasing the sensitivity of the calcium sensing receptor to extracellular calcium. The reduction in PTH is associated with a concomitant decrease in serum calcium levels.
Reductions in PTH levels correlate with cinacalcet concentration.
After steady state is reached, serum calcium concentrations remain constant over the dosing interval.
After oral administration of cinacalcet, maximum plasma cinacalcet concentration is achieved in approximately 2 to 6 hours. Based on between-study comparisons, the absolute bioavailability of cinacalcet in fasted subjects has been estimated to be about 20-25%. Administration of cinacalcet with food results in an approximate 50–80% increase in cinacalcet bioavailability. Increases in plasma cinacalcet concentration are similar, regardless of the fat content of the meal.
At doses above 200 mg, the absorption was saturated probably due to poor solubility.
The volume of distribution is high (approximately 1,000 litres), indicating extensive distribution. Cinacalcet is approximately 97% bound to plasma proteins and distributes minimally into red blood cells.
After absorption, cinacalcet concentrations decline in a biphasic fashion with an initial half-life of approximately 6 hours and a terminal half-life of 30 to 40 hours. Steady state levels of cinacalcet are achieved within 7 days with minimal accumulation. The pharmacokinetics of cinacalcet does not change over time.
Cinacalcet is metabolised by multiple enzymes, predominantly CYP3A4 and CYP1A2 (the contribution of CYP1A2 has not been characterised clinically). The major circulating metabolites are inactive.
Based on in vitro data, cinacalcet is a strong inhibitor of CYP2D6, but is neither an inhibitor of other CYP enzymes at concentrations achieved clinically, including CYP1A2, CYP2C8, CYP2C9, CYP2C19, and CYP3A4 nor an inducer of CYP1A2, CYP2C19 and CYP3A4.
After administration of a 75 mg radiolabelled dose to healthy volunteers, cinacalcet was rapidly and extensively metabolised by oxidation followed by conjugation. Renal excretion of metabolites was the prevalent route of elimination of radioactivity. Approximately 80% of the dose was recovered in the urine and 15% in the faeces.
The AUC and Cmax of cinacalcet increase approximately linearly over the dose range of 30 to 180 mg once daily.
Soon after dosing, PTH begins to decrease until a nadir at approximately 2 to 6 hours post-dose, corresponding with cinacalcet Cmax. Thereafter, as cinacalcet levels begin to decline, PTH levels increase until 12 hours post-dose, and then PTH suppression remains approximately constant to the end of the once daily dosing interval. PTH levels in cinacalcet clinical trials were measured at the end of the dosing interval.
There are no clinically relevant differences due to age in the pharmacokinetics of cinacalcet.
The pharmacokinetic profile of cinacalcet in patients with mild, moderate, and severe renal insufficiency, and those on haemodialysis or peritoneal dialysis is comparable to that in healthy volunteers.
Mild hepatic impairment did not notably affect the pharmacokinetics of cinacalcet. Compared to subjects with normal liver function, average AUC of cinacalcet was approximately 2-fold higher in subjects with moderate impairment and approximately 4-fold higher in subjects with severe impairment. The mean half-life of cinacalcet is prolonged by 33% and 70% in patients with moderate and severe hepatic impairment, respectively. Protein binding of cinacalcet is not affected by impaired hepatic function. Because doses are titrated for each subject based on safety and efficacy parameters, no additional dose adjustment is necessary for subjects with hepatic impairment.
Clearance of cinacalcet may be lower in women than in men. Because doses are titrated for each subject, no additional dose adjustment is necessary based on gender.
The pharmacokinetics of cinacalcet was studied in paediatric patients with ESRD receiving dialysis aged 3 to 17 years of age. After single and multiple once daily oral doses of cinacalcet, plasma cinacalcet concentrations (Cmax and AUC values after normalisation by dose and weight) were similar to those observed in adult patients.
A population pharmacokinetic analysis was performed to evaluate the effects of demographic characteristics. This analysis showed no significant impact of age, sex, race, body surface area, and body weight on cinacalcet pharmacokinetics.
Clearance of cinacalcet is higher in smokers than in non-smokers, likely due to induction of CYP1A2-mediated metabolism. If a patient stops or starts smoking, cinacalcet plasma levels may change and dose adjustment may be necessary.
Cinacalcet was not teratogenic in rabbits when given at a dose of 0.4 times, on an AUC basis, the maximum human dose for secondary HPT (180 mg daily). The non-teratogenic dose in rats was 4.4 times, on an AUC basis, the maximum dose for secondary HPT. There were no effects on fertility in males or females at exposures up to 4 times a human dose of 180 mg/day (safety margins in the small population of patients administered a maximum clinical dose of 360 mg daily would be approximately half those given above).
In pregnant rats, there were slight decreases in body weight and food consumption at the highest dose. Decreased foetal weights were seen in rats at doses where dams had severe hypocalcaemia. Cinacalcet has been shown to cross the placental barrier in rabbits.
Cinacalcet did not show any genotoxic or carcinogenic potential. Safety margins from the toxicology studies are small due to the dose-limiting hypocalcaemia observed in the animal models. Cataracts and lens opacities were observed in the repeat dose rodent toxicology and carcinogenicity studies, but were not observed in dogs or monkeys or in clinical studies where cataract formation was monitored. Cataracts are known to occur in rodents as a result of hypocalcaemia.
In in vitro studies, IC50 values for the serotonin transporter and KATP channels were found to be 7 and 12-fold greater, respectively, than the EC50 for the calcium-sensing receptor obtained under the same experimental conditions. The clinical relevance is unknown, however, the potential for cinacalcet to act on these secondary targets cannot be fully excluded.
In toxicity studies in juvenile dogs, tremors secondary to decreased serum calcium, emesis, decreased body weight and body weight gain, decreased red cell mass, slight decreases in bone densitometry parameters, reversible widening of the growth plates of long bones, and histological lymphoid changes (restricted to the thoracic cavity and attributed to chronic emesis) were observed. All of these effects were seen at a systemic exposure, on an AUC basis, approximately equivalent to the exposure in patients at the maximum dose for secondary HPT.
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