Caplacizumab is a humanised bivalent Nanobody that consists of two identical humanised building blocks (PMP12A2hum1), genetically linked by a three-alanine linker, targeting the A1-domain of von Willebrand factor and inhibiting the interaction between von Willebrand factor and platelets. As such, caplacizumab prevents the ultralarge von Willebrand factor-mediated platelet adhesion, which is characteristic of aTTP. It also affects the disposition of von Willebrand factor, leading to transient reductions of total von Willebrand factor antigen levels and to concomitant reduction of factor VIII:C levels during treatment.
The pharmacologic effect of caplacizumab on target inhibition was assessed using two biomarkers for von Willebrand factor activity; ristocetin-induced platelet aggregation (RIPA) and ristocetin cofactor (RICO). Full inhibition of von Willebrand factor-mediated platelet aggregation by caplacizumab is indicated by RIPA and RICO levels dropping below 10% and 20%, respectively. All clinical studies with caplacizumab demonstrated rapid decreases in RIPA and/or RICO levels after the start of the treatment, with recovery to baseline levels within 7 days of discontinuation. The 10 mg subcutaneous dose in patients with aTTP elicited full inhibition of von Willebrand factor-mediated platelet aggregation, as evidenced by RICO levels of < 20% throughout the treatment period.
The pharmacologic effect of caplacizumab on target disposition was measured using von Willebrand factor antigen and factor VIII clotting activity (factor VIII:C) as biomarkers. Upon repeated administration of caplacizumab, a decrease of 30-50% in von Willebrand factor antigen levels was observed in clinical studies, reaching a maximum within 1-2 days of treatment. Because von Willebrand factor acts as a carrier for factor VIII, reduced von Willebrand factor antigen levels resulted in a similar reduction in factor VIII:C levels. The reduced von Willebrand factor antigen and FVIII:C levels were transient and returned to baseline upon cessation of treatment.
The pharmacokinetics of caplacizumab have been investigated in healthy subjects after single intravenous infusions and after single and repeated subcutaneous injections. Pharmacokinetics in patients with aTTP were investigated upon single intravenous and repeated subcutaneous injections. Pharmacokinetics of caplacizumab appear as non-dose proportional, as characterized by targetmediated disposition. In healthy volunteers receiving 10 mg caplacizumab subcutaneoulsy once daily, the maximum concentration was observed at 6-7 hours post-dose and steady-state was reached following the first administration, with minimal accumulation.
After subcutaneous administration, caplacizumab is rapidly and almost completely absorbed (estimated F>0.901) in the systemic circulation.
After absorption, caplacizumab binds to the target and distributes to well perfused organs. In patients with aTTP the central volume of distribution was estimated at 6.33 L.
The pharmacokinetics of caplacizumab depend on the expression of the target von Willebrand factor. Higher levels of von Willebrand factor antigen, such as in patients with aTTP, increase the fraction of drug-target complex retained in the circulation. The t1/2 of caplacizumab is, therefore, concentrationand target level-dependent. Target-bound caplacizumab is assumed to be catabolised within the liver, whereas unbound caplacizumab is assumed to be renally cleared.
The pharmacokinetics of caplacizumab were determined using a population pharmacokinetic analysis on pooled pharmacokinetic data. Body weight was allometrically included in the model. Differences in the different subpopulations were investigated. In studied populations; gender, age, blood group and race did not affect the pharmacokinetics of caplacizumab.
No formal study of the effect of hepatic or renal impairment on the pharmacokinetics of caplacizumab has been conducted. In the population PK/PD model, renal function (CRCL) had a statistically significant effect resulting in limited increase in predicted exposure (AUCss) in severe renal impairment. In the clinical studies of patients with TTP, those with renal impairment did not show additional risk of adverse events.
Consistent with its mode of action, toxicology studies of caplacizumab have shown an increased bleeding tendency in guinea pigs (haemorrhagic subcutaneous tissue at the injection sites) and cynomolgus monkeys (haemorrhagic subcutaneous tissue at the injection sites, nose bleed, exaggerated menstrual bleeding, haematoma at sites of animal handling or experimental procedures, prolonged bleeding at injection sites). Furthermore, pharmacology-related decreases of von Willebrand factor antigen, and consequently factor VIII:C, were noted in cynomolgus monkeys and, to a lesser extent for factor VIII:C, in guinea pigs.
An embryo-foetal development study was conducted in guinea pigs, with no reported signs of toxicity. A follow-up toxicokinetic study in pregnant guinea pigs assessed exposure of caplacizumab in the dams and foetuses. The results indicated exposure to caplacizumab in dams and, to a much lesser extent, foetuses, with no reported effects on foetal development. Foetal exposure to caplacizumab in primates and humans remains uncertain, as proteins lacking an Fc portion are not thought to freely pass the placental barrier.
No studies have been performed to evaluate the mutagenic potential of caplacizumab, as such tests are not relevant for biologicals. Based on a carcinogenicity risk assessment, dedicated studies were not deemed necessary.
Dedicated animal studies assessing the effects of caplacizumab on male and female fertility have not been performed. In repeat-dose toxicity tests in cynomolgus monkeys, no impact of caplacizumab on fertility parameters in male (testicular size, sperm function, histopathological analysis of testis and epididymis) and female (histopathological analysis of reproductive organs, periodic vaginal cytology) animals was observed.
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