Minibodies to maximize platelets?

Comment on Orita et al, page 562

Orita and colleagues have created novel c-mpl agonists from antireceptor antibodies. Such “minibodies” may represent a viable alternative for turning on platelet production in thrombocytopenic patients.

Clinically significant thrombocytopenia is a common problem in chemotherapy-treated patients. The mainstay of treatment for these patients is platelet transfusion. Although these transfusions can be lifesaving, they may be associated with transfusion reactions, and transmission of viral or bacterial pathogens.¹  In addition, patients (particularly those receiving multiple transfusions) may become refractory to platelet transfusions due to alloimmunization. Additional strategies for the treatment of thrombocytopenic patients would be beneficial.

Thrombopoietin (TPO, c-mpl ligand) is a lineage-specific regulator of megakaryopoiesis and platelet production. In humans, 2 recombinant forms of TPO (pegylated recombinant human megakaryocyte growth and development factor [PEG-rhMGDF] and recombinant human TPO [rhTPO]) have been used to treat thrombocytopenia.²  TPO acts by binding to c-mpl, a type I hematopoietic transmembrane receptor. Efficacy has been demonstrated for TPO in the treatment of thrombocytopenia in patients receiving nonmyeloablative chemotherapy. In contrast, TPO is not effective for patients receiving myeloablative chemotherapeutic regimens. In addition, TPO treatment may be complicated by the production of neutralizing and nonneutralizing antibodies that cross-react with native TPO.FIG1 

In this issue of Blood, Orita and colleagues have taken a novel approach to create alternative c-mpl agonists. They began by identifying c-mpl agonist antibodies, antibodies that activate instead of block receptor function. However, they found that even antibodies with reasonably high affinity displayed weak agonist activity. Hypothesizing that the weak agonism of these antibodies may be due to inefficient receptor dimerization because of steric hindrance, they created 2 types of dimeric antibody fragments, or minibodies, in an attempt to improve c-mpl activation. One of these (diabody) is a noncovalent dimer, while the other (sc(FV)2) is covalent (see figure). Minibodies from 2 antibodies, VB22B and TA136, were characterized.

The first minibody, VB22B sc(Fv)2, increased proliferation of M-07e cells (a TPO-dependent, megakaryocytic cell line) as well as CD41⁺ cells from a population of human bone marrow progenitor cells. This minibody displayed potency similar to TPO. VB22B sc(Fv)2 was also shown to have in vivo activity in normal cynomolgus monkeys. Although increased platelet counts were seen in monkeys treated with either VB22B sc(Fv)2 or TPO, rebound thrombocytopenia was seen only with TPO. Although relatively few animals were used, these experiments are exciting, and support the performance of more extensive studies in vivo of this novel c-mpl agonist.

Interestingly, TA136 sc(Fv)2, another minibody, was found to increase proliferation of TPO-dependent cell lines derived from 2 patients with congenital amegakaryocytic thrombocytopenia (CAMT), a disorder caused by mutant c-mpl. The agonist activity of TA136 sc(Fv)2 was greatly improved over TPO at these mutant receptors. This suggests a paradigm in which antibody fragments could be designed to treat a variety of receptor-based diseases.

One implication of the work by Orita et al, is that alternative c-mpl agonists may repre sent a new approach to the treatment of thrombocytopenia. However, another inter esting aspect of this work is the creation and use of the minibodies themselves. Minibodies could be created against a variety of growth and differentiation receptors that require dimerization for activation. Creation of such molecules could be useful in the treatment of inflammatory and/or malignant disorders. ▪