Immunotoxins for hairy cell leukemia
lysine residue, (5) undergoing reduction of a disulfide bond holding the two fragments together, (6) transporting of the carboxyl terminal portion of the toxin from the transreticular Golgi to the endoplasmic reticulum by the KDEL receptor, (7) translocation to the cytosol, and finally (8) catalytic adenosine diphosphate (ADP) ribosylation of elonga- tion factor II, leading to apoptotic cell death. Thus, recombinant immunotoxins kill target cells selec- tively without requiring immune mechanisms, and their mechanism is markedly different from che- motherapy including purine analogs.
LMB-2 targeting CD25+ hairy cell leukemia
LMB-2 was the first immunotoxin with reported efficacy in HCL. To target CD25, the variable domains VH and VL of the anti-CD25 mAb anti- Tac were fused together with a peptide linker (G4S)3 and VL was fused to PE38 via a short connector ASGGPE (Figure 1). In a phase I trial conducted in patients with hematologic malignancies, four out of four patients with HCL responded, one completely (CR) and three partially (PR) [6,7]. Responses were
83
observed in other hematologic malignancies, and activity has been observed in a follow-up phase II trial in HCL. Although only 10% of patients with HCL have the CD25-negative variant HCLv, because HCLv is primarily refractory to cladribine, patients with HCLv are overrepresented in the relapsed/ refractory population. Thus, CD22, which is strongly positive in essentially all cases of HCL, was targeted.
BL22, targeting CD22
BL22 contains a disulfide bond linking VH and VL, with VH fused to PE38 (Figure 1). In a phase I trial of 31 patients with cladribine relapsed/refractory HCL, BL22 achieved 19 (62%) CRs and six (19%) PRs, an overall response rate (ORR) of 81% [8,9]. In this trial, three patients had HCLv and all had CR, and although two of the three relapsed, both were returned to CR with salvage BL22. The most common serious toxicity was a completely reversible hemolytic uremic syndrome (HUS), in four patients. HUS presented with hematuria, hemoglobinuria, thrombocytopenia, and renal insufficiency by day 8 of cycle 2–3, and completely resolved within several
Figure 1. Molecules under investigation for HCL. Recombinant immunotoxins are 63 kDa and contain an Fv domain fused via the C3 connector (ASGGPE, light blue) to amino acids 253–364 and 381–613 of Pseudomonas exotoxin. Domain II (yellow), containing amino acids 253–364, is the translocating domain, domain III (red), containing amino acids 395–613, is the ADP ribosylating domain, and part of domain Ib (amino acids 381–394) separates domains II and III. Anti-Tac(Fv)-PE38 (LMB-2), targeting CD25, contains a (G4S)3 linker between VH and VL, and VL is fused to PE38. In BL22 and HA22, VL and VH are connected by a disulfide bond using cysteine residues replacing Arg44 of VH and Gly100 of VL. HA22, also called moxetumomab pasudotox or CAT-8015, is a high-affinity variant of BL22 containing THW replacing SSY at positions 100, 100a, and 100b of VH. Rituximab is an anti-CD20 chimeric antibody containing murine variable domains and human constant (yellow and pink) domains. Bendamustine and pentostatin are purine analogs, and bendamustine also contains a nitrogen mustard group (blue) which provides alkylator activity.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122