Earlier this year, the Food and Drug Administration approved Cablivi (caplacizumab-yhdp) (Sanofi Genzyme, Cambridge, Mass.) for the treatment of acquired thrombotic thrombocytopenic purpura (TTP), making it the first medication specifically indicated for the treatment of TTP.
The approval of caplacizumab and the clinical trial results that approval is based on are the most promising developments in the treatment of TTP since the introduction of plasma exchange (PE) therapy. However, many questions remain about how to best administer caplacizumab, specifically, which patients should receive it? Should all TTP patients start on caplacizumab therapy or should it be limited to patients with histories of TTP or those slow to respond to standard therapy with PE and immunosuppression?
TTP is a rare thrombotic microangiopathy characterized by thrombocytopenia and microangiopathic hemolytic anemia caused by the inhibition of ADAMTS13, a metalloproteinase, which cleaves large-molecular-weight von Willebrand factor (vWF) multimers. Caplacizumab is a humanized bivalent, variable domain-only immunoglobulin fragment. The drug targets the A1 domain of vWF and inhibits the binding between vWF and the platelet glycoprotein Ib-IX-V receptor, preventing the formation of the microvascular thrombi and platelet loss associated with TTP.
FDA approval of caplacizumab came shortly after the publication of the results of the HERCULES trial in the New England Journal of Medicine by Marie Scully, MD, and her colleagues ().
HERCULES was an international phase 3, double blinded, placebo-controlled, randomized study designed to evaluate the efficacy and safety of caplacizumab. In total, 145 patients participated in the trial. Caplacizumab or placebo were given in addition to standard therapy of plasma exchange (PE) and immunosuppression. Caplacizumab or placebo were administered as an intravenous loading dose prior to the first PE after randomization and subcutaneously once daily until 30 days after the last PE. All patients received daily PE until 2 days after platelet count normalization.
The primary measure of the study was the time to platelet count response of greater than 150 x 109/L following the cessation of daily PE. Secondary measures included TTP-related death; TTP relapse; major thromboembolic events; proportion of subjects with refractory TTP; normalization of organ damage markers including lactate dehydrogenase, cardiac troponin I, and serum creatinine; and other adverse events.
The authors found that the median time of normalization of the platelet count was shorter in the caplacizumab group, compared with placebo, with the caplacizumab group being 1.55 times more likely to have a normalized platelet count at any given time point in the study. While statistically significant differences were identified in the rate of platelet normalization, the median number of days of PE until normalization was only 2 days less in the caplacizumab group (five treatments) than in the placebo group (seven treatments), which may not be clinically significant for the treating physician.
In fact, the secondary endpoints of the study seem much more clinically promising in the treatment of TTP. The composite rate of TTP-related death, TTP recurrence, or major thromboembolic events during the treatment period was significantly lower in the caplacizumab group (12%) versus placebo (49%). No TTP-related deaths occurred in the caplacizumab group. The caplacizumab group was also statistically less likely to have a TTP exacerbation, defined as disease recurrence within 30 days from the last PE, than the placebo group.
End organ damage serum markers also improved faster in the caplacizumab group, although there was no significant difference between groups. Overall hospitalization (median of 9 vs. 12 days) and ICU stays (median of 3 vs. 5 days) were shorter in the caplacizumab group, compared with the placebo group.
While several relapses, defined as disease recurrence after 30 days from the last PE, occurred in the caplacizumab group, the relapses were only found in patients with ADAMTS13 activity of less than 10% at the end of the treatment period. Mild side effects, such as mucocutaneous bleeding were more frequent in the caplacizumab group. No major bleeding complications were observed.
The HERCULES trial generates more questions about the role of ADAMTS13 activity testing to monitor treatment response and to make therapy decisions. Extremely low ADAMTS13 activity levels at the cessation of therapy may be a sign of treatment inadequacy and may warrant closer follow-up of at-risk patients on caplacizumab.
Sanofi Genzyme estimates that the U.S. list price will be approximately $270,000 for a standard treatment course, according to afrom the company. Whether payers will add it to formularies remains uncertain, but the high drug cost may be countered by potential savings in the reduction of hospital and ICU days with caplacizumab therapy. Sanofi Genzyme will also have a patient support program for eligible patients.
Caplacizumab has been approved in Europe since August 2018, but is not readily available in the United States. Given the dearth of clinical experience with the drug outside of the TITAN and HERCULES trials, strong recommendations for when and how to initiate therapy remain elusive.
As caplacizumab is further introduced into clinical practice, more studies are needed to identify which patient groups will benefit most from therapy. The current data for caplacizumab shows that it will be used as an adjunct to standard PE therapy, rather than as a replacement. How the drug is used in combination with current TTP treatments – such as corticosteroids, rituximab, bortezomib, vincristine, N-acetylcysteine, and splenectomy – should be evaluated to identify which treatment combinations not only improve platelet counts, but also reduce mortality and morbidity while remaining cost effective.
Dr. Ricci is a staff physician and Apheresis Director at the Taussig Cancer Institute at the Cleveland Clinic. She reported having no conflicts of interest.