Asfotase alfa, a human recombinant tissue-nonspecific alkaline phosphatase-Fc-deca-aspartate fusion protein with enzymatic activity, promotes mineralisation of the skeleton in patients with hypophosphatasia.
Hypophosphatasia is a rare, severe, and potentially fatal, genetic disorder caused by loss-of-function mutation(s) in the gene encoding tissue non-specific alkaline phosphatase. Hypophosphatasia is associated with multiple bone manifestations including rickets/osteomalacia, altered calcium and phosphate metabolism, impaired growth and mobility, respiratory compromise that may require ventilation, and vitamin B6-responsive seizures.
Pharmacokinetics of asfotase alfa were evaluated in a 1-month, multicenter, open-label, dose- escalating, study in adults with hypophosphatasia. Cohort 1 (n=3) of the study received asfotase alfa 3 mg/kg intravenously the first week followed by 3 doses at 1 mg/kg subcutaneous at weekly intervals from weeks 2 to 4. Cohort 2 (n=3) received asfotase alfa 3 mg/kg intravenously the first week followed by 3 doses at 2 mg/kg subcutaneous at weekly intervals from weeks 2 to 4. After the 3 mg/kg for 1.08 hours intravenous infusion, the median time (Tmax) ranged between 1.25 to 1.50 hours, and the mean (SD) Cmax ranged between 42694 (8443) and 46890 (6635) U/L over the studied cohorts. The absolute bioavailability after the first and third subcutaneous administration ranged from 45.8 to 98.4%, with median Tmax ranging between 24.2 to 48.1 hours. After the 1 mg/kg weekly subcutaneous administration in Cohort 1 the mean (SD) AUC over the dosing interval (AUCτ) was 66034 (19241) and 40444 (N=1) U*h/L following the first and the third dose, respectively. After the 2 mg/kg weekly subcutaneous administration in Cohort 2 the mean (SD) AUCτ was 138595 (6958) and 136109 (41875) following the first and the third dose, respectively.
Pharmacokinetic data from all asfotase alfa clinical trials were analysed using population pharmacokinetic methods. The pharmacokinetic variables characterized by population pharmacokinetic analysis represent the overall hypophosphatasia patient population with age range from 1 day to 66 years, subcutaneous doses of up to 28 mg/kg/week and a range of disease onset cohorts. Twenty five percent (15 out of 60) of the overall patient population was adult (>18 years) at baseline. The absolute bioavailability and absorption rate following subcutaneous administration were estimated to be 0.602 (95% CI: 0.567, 0.638) or 60.2% and 0.572 (95%CI: 0.338, 0.967)/day or 57.2%, respectively. The central and peripheral volumes of distribution estimates for a patient with body weight of 70 kg (and 95% CI) were 5.66 (2.76, 11.6) L and 44.8 (33.2, 60.5) L, respectively. The central and peripheral clearance estimates for a patient with body weight of 70 kg (and 95% CI) were 15.8 (13.2, 18.9) L/day and 51.9 (44.0, 61.2) L/day, respectively. The extrinsic factors affecting asfotase alfa pharmacokinetic exposures were formulation specific activity and total sialic acid content. The average ± SD elimination half-life following subcutaneous administration was 2.28 ± 0.58 days.
In adult patients with pediatric-onset HPP, the pharmacokinetics of asfotase alfa at doses of 0.5, 2 and 3 mg/kg administered three times per week was consistent with those observed in pediatric patients with pediatric-onset HPP, and thus supported the approved dose of 6 mg/kg per week in treating adult patients with pediatric-onset HPP.
Based on the results of population pharmacokinetic analysis it was concluded that asfotase alfa exhibits linear pharmacokinetic up to subcutaneous doses of 28 mg/kg/week. The model identified body weight to affect asfotase alfa clearance and volume of distribution parameters. It is expected that pharmacokinetic exposures will increase with body weight. The impact of immunogenicity on asfotase alfa pharmacokinetic varied over time due to the time varying nature of immunogenicity and overall was estimated to decrease pharmacokinetic exposures by less than 20%.
In nonclinical safety testing in rats, no body system-specific adverse effects were noted at any dose or route of administration.
Dose – and time-dependent acute injection reactions that were transient and self-limiting were noted in rats at intravenous use doses of 1 to 180 mg/kg.
Ectopic calcifications and injection site reactions were observed in monkeys when asfotase alfa was administered subcutaneously at daily doses up to 10 mg/kg through 26 weeks. These effects were restricted to injection sites and were partially or completely reversible. There was no evidence of ectopic calcification observed in any other tissues examined.
Preclinical data reveal no special hazard for humans based on conventional studies of safety pharmacology, repeated dose toxicity or toxicity to reproduction and development. However, in pregnant rabbits administered intravenous doses of up to 50 mg/kg/day asfotase alfa, anti-drug antibodies were detected in up to 75% of animals which could affect the detection of reproductive toxicity.
No animal studies have been conducted to evaluate the genotoxic and carcinogenic potential of asfotase alfa.
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