How an engineered therapy could replace the need for potentially harmful plasma infusions

KEYWORDS:

• TTP
• ADAMTS 13
• fresh frozen plasma
• recombinant therapy

Thrombotic thrombocytopenic purpura (TTP) is an acute life-threatening disorder associated with significant morbidly and mortality, affecting primarily young adults. It presents with thrombocytopenia, hemolytic anemia and microvascular thrombosis resulting in multi-organ involvement. At the end of the 1990s, two groups identified that the condition was the result of a deficiency of an enzyme required to cleave von Willebrand Factor [1,2]. This was recognized in patients we now know had immune-mediated, antibody driven TTP (iTTP), but also with a congenital deficiency of the protein. It was not until 2001 that von Willebrand Factor-cleaving protein was identified as the metalloproteinase, ADAMTS 13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) [3].

ADAMTS 13 is required to cleave the von Willebrand factor (VWF). In TTP, when there is asevere deficiency of ADAMTS 13, VWF cannot be broken down and in particular, there is an excess of ultra large VWF. The result is excess platelet binding, and microthrombi formation, made up of VWF and platelets, which is evident in most organs. It is the point of critical microthrombi that results in clinical symptoms.

Currently, guidelines require intensive treatment of patients with iTTP with plasma exchange, providing a source of ADAMTS 13, required to deliver high volumes, as well as aiding removal of ADAMTS 13 antibodies. Initially at 60ml/kg per exchange, on improvement of the platelet count, this can be reduced to 40ml/kg, representing 1.5 and single volume exchanges. Plasma exchange is continued daily to complete remission. This is confirmed by the normalization of platelet count, above the laboratory range, typically 150x 10⁹/L, normalization of lactate dehydrogenase (LDH) levels and more importantly, confirmation that ADAMTS 13 levels have increased. In conjunction with plasma exchange, immunosuppressive therapy is used, including steroids and anti-CD20 monoclonal therapy [4]. The use of immunosuppressive therapy is to aid removal of the ADAMTS 13 antibodies. However, even in those cases that are diagnosed promptly and receive treatment, the risk of dying is highest in the first few days. Approximately 20% of cases are refractory and there is a significant risk of exacerbations during an acute TTP episode. Therefore, there remains a significant gap from presentation until current therapies induce a clinical remission.

iTTP makes up the majority of TTP cases, but stil, the incidence is that of an ultra-rare disease, 4–6/million of the population. Congenital TTP was originally thought to occur primarily in the neonatal period and childhood. However, over the last few years, we now know that most cases present later in adulthood, usually associated with a significant trigger e.g. pregnancy [5,6]. How we treat congenital TTP is evolving. Those presenting in early life typically receive prophylaxis, while in later years, other than in pregnancy or to cover a procedure, widespread prophylaxis is not common. Prophylaxis is primarily with plasma, the volume and frequency varying, but typically 10–15 ml fresh frozen plasma every 1–4 weeks. Intermediate purity Factor VIII concentrates are used e.g. BPL 8Y [7]or Koate DVI (in the US) [8].

Both iTTP and congenital cases are at risk from relapses and therefore, currently, further plasma is required to replenish the missing enzyme, ADAMTS 13. While the volumes of plasma to normalize routine laboratory parameters in congenital TTP are not as large, iTTP use approximately 40 l of plasma per episode of acute TTP.

We need to consider the lessons learnt from other rare hematological disorders requiring replacement of a missing enzyme, most notably, Hemophilia A. This is the X linked deficiency of Factor VIII and levels <1% is associated with severe bleeding, primarily targeting joints and muscles. In the late 1950s and 1960s, the main treatment for hemophilia was fresh frozen plasma. However, large volumes were required to stop bleeding. Aside from the need to be admitted and treated in hospital, delays in therapy resulted in crippling joint defects. By the 1970s, the availability of lyophilized Factor VIII bottles of concentrate allowed for home therapy and more accurate dosing. However, the hemophilia community paid a terrible price. The discovery of hepatitis virus transfer, subsequently identified as B and C and the passage of HIV within blood products resulted in morbidity and mortality in hemophilia patients as a result of therapy, while controlling their bleeding disorder. From the mid-1980s, fractionated factor VIII products underwent heat treatment, solvent detergent treatment or sterilization, to prevent further viral transfer. In the mid-1980s, the Factor VIII gene was cloned and by 1992, two pharmaceutical companies had licensed recombinant Factor VIII products for use in Haemophilia A.

The Factor VIII story is not dissimilar to that of TTP, in that TTP patients still require large volumes of fresh frozen plasma for treatment. Some countries use virally inactivated plasma e.g. Octaplas, which also has a prion reduction step (Octaplas LG), but this is not a ubiquitous practice. While plasma is screened for known pathogens for which there are screening tests, there still remains a considerable potential risk for those that have not been identified or can t be screen for, such as prions. Yet fresh frozen plasma is the primary source of ADAMTS 13 replacement. The treatment of TTP is decades behind that of other enzyme replacement disorders.

Given the large volumes of plasma required, there remain significant risks in treating patients. In the majority of cases, insertion of a large central venous catheter is required to undertake the procedure of plasma exchange, reactions including anaphylaxis, that may exacerbate an already acute situation and volume overload in those receiving infusions. For those who have reactions, even with premedication, it may mean an inability to give adequate volumes of treatment. The intermediate purity factor VIII products are a potential alternative, but the quantity of ADAMTS 13 is negligible in comparison and the products, by virtue of their name, contain other coagulation factors, which are not required in TTP- a risk also with FFP (fresh frozen plasma).

So what is the future for TTP?

The two important developments are:

1. Recombinant ADAMTS 13. The phase 1 first in human study, following a single dose of BAX930, was safe and demonstrated important pharmacokinetic data in congenital TTP [9]. Currently, a phase III study is underway (NCT03393975) and a phase II study planned for immune-mediated TTP. The benefits of this therapy are enormous. Aside from more frequent, correct dosing in congenital TTP cases, the aim is to avoid the need for immune-mediated TTP cases to have to receive PEX (plasma exchange). One of the biggest potential risks is the development of antibodies/inhibitors to recombinant Factor VIII in hemophilia A and this will need to be monitored closely with recombinant ADAMTS 13 therapy in TTP.

2. The phase II results have been published [10] and phase III study completed [11] in iTTP patients receiving the nanobody. Caplacizumab targets the A1-domain of VWF, inhibiting the interaction between ultra-large VWF and platelet GpIb-IX-V. Studies in acute, acquired TTP, prevention of platelet binding to VWF, results in a faster increase in platelet count, prevention of further microvascular thrombi and time for immunotherapy to become effective. This decreases the need for such intensive plasma therapy and volume of plasma used during an acute TTP episode. However, there is also a likely benefit to be gained in congenital TTP cases.

Therefore, what can we expect for patients with TTP in the next 5–10 years? First and foremost, the use of plasma for the treatment of TTP should be redundant. The caveat is that the diagnosis of iTTP is confirmed promptly with ADAMTS 13 analysis. Recombinant ADAMTS 13 will replace the missing enzyme and should be the mainstay of treatment in patients with immune-mediated and congenital TTP. The dose in iTTP will need to be much higher to overcome the effect of Anti ADAMTS 13 antibodies; this will be investigated in specific clinical trials. The role of nanobodies and monoclonal therapy in conjunction with the rADAMTS 13 should ensure that patients have a quicker response to treatment, reducing morbidity, improving mortality and preventing the long-term sequelae of acute and subacute disease. In congenital TTP, recombinant ADAMTS 13 will provide pure protein delivered at small volumes, and the ability to achieve higher peak levels of the missing enzyme.

Declaration of interest

M Scully has disclosed speakers fees from Ablynx, Shire, Alexion, Novartis and advisory boards with these companies. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was not funded.