Relapsed hairy cell leukemia 33
death by apoptosis [15,16]. Bacterial toxins, which
BL22 targeting CD22
are more often used to fuse to ligands, are naturally
made by bacteria in single-chain form, composed of In phase I testing, 31 of the 46 patients had HCL
domains for binding and ADP-ribosylation at oppo- relapsed or refractory after purine analogs, and of
site ends, and a translocation domain in between these 31 patients there were 19 (61%) CRs and 6
[16–19]. The orientation of the domains are opposite (19%) PRs [34,35]. A completely reversible form of
in PE and DT, with the binding domain at the amino hemolytic uremic syndrome (HUS) was observed in
terminus of PE and at the carboxyl terminus of DT. four (13%) of the patients, but only during retreat-
They intoxicate cells by binding to the cell surface, ment with a second or third cycle. However, most
undergoing internalization, unfolding within an (58%) of the CRs were achieved with one cycle,
acidic vesicle, undergoing proteolytic cleavage within suggesting that retreatment was not always necessary
the translocating domain, and translocating to the or advisable. Therefore, in the phase II trial, patients
cytosol where EF-2 is inactivated by ADP-ribosyla- were observed without retreatment if they achieved
tion [19]. Recombinant toxins are produced by CR or at least the complete blood count (CBC)
replacing the binding domain with a cancer cell- values required for CR, and if not, they were
binding ligand. In denileukin diftitox, approved for retreated at a lower dose level. One cycle of BL22
relapsed and refractory cutaneous T-cell lymphoma, induced CR in 25% of 36 patients, and by retreating
human interleukin-2 replaces the binding domain of the 56% of patients who were eligible for retreat-
DT at the carboxyl terminus [20–22]. In recombi- ment, the CR rate rose to 47%, with an overall
nant immunotoxins, the cell-binding ligand is an Fv response rate of 72% [36]. Compared to the phase I
fragment, a recombinant antibody containing the trial, the dose-limiting HUS rate was less than half
variable domains of a Mab [23]. In the recombinant (6% vs. 13%) and the immunogenicity rate less than
immunotoxin LMB-2, the variable domains of the one-third (11% vs. 35%). The median CR duration
anti-CD25 Mab are fused together by a peptide was 5–46þ(median 22þ) months, with 76% of 17
linker and then fused to the amino terminus of PE38, CRs ongoing [36]. Response was not significantly
a fragment of PE which is missing its binding domain related to length of prior purine analog response,
[19,24]. In the recombinant immunotoxin BL22, the but was significantly higher in patients with
variable domains of the MAb RFB4 are fused to spleens5200 mm in maximal dimension than in
PE38, but the Fv is stabilized with an engineered patients with either larger spleens or prior splenect-
disulfide bond rather than a peptide linker [25–27]. omy. Pharmacokinetic studies showed significant
inverse relationships between area under the curve
and tumor burden measured by either the log of the
LMB-2 targeting CD25
HCL count in the blood, or the spleen size. It was
LMB-2 was the first recombinant immunotoxin concluded that BL22 is highly active in HCL despite
reported to be of benefit for HCL, with four prior purine analog treatment and resistance, and
(100%) responses out of four patients treated as that experimental therapy should be considered in
part of a phase I trial, one of whom had a durable CR patients prior to removing the spleen or allowing it to
[28,29]. The three patients with PR had inadequate become massive in size.
treatment due to pneumonia, dose-limiting toxicity
and immunogenicity, suggesting that significant
HA22, a high affinity mutant of BL22
clinical activity might be observed in a phase II
setting. Unlike purine analogs, myelosuppression Patients with chronic lymphocytic leukemia (CLL)
and severe lymphopenia were not observed with were not as sensitive to BL22, attributed to much
LMB-2 [28,29], which was expected since normal T- lower expression of CD22 on CLL compared to
and B-cells have insufficient CD25 expression to be HCL cells [35]. To allow a higher percentage of
sensitive [30,31]. The most common toxicities of bound immunotoxin to be internalized by CD22
LMB-2 included non-dose limiting hypoalbumine- rather than disassociating from this antigen, the off-
mia, transaminase and alkaline phosphatase eleva- rate of BL22 was lowered by mutagenesis within the
tions, and fever. A disadvantage of targeting CD25 in third complementarity determining region of the
HCL is that 10% of patients have a variant (HCLv) heavy chain. The resulting anti-CD22 recombinant
in which CD25 is absent [32,33], and this variant is immunotoxin HA22 contained three amino acid
over-represented by patients with relapsed/refractory mutations and bound with 15-fold higher affinity
disease after purine analogs. Both HCLv and classic and lower off-rate to CD22, and had improved
HCL uniformly express high levels of CD22, and cytotoxicity [37–39]. HA22 is undergoing phase I
have more recently been targeted by the recombinant testing in HCL, CLL and non-Hodgkin lymphoma.
immunotoxins BL22. The HCL trial has neared completion with excellent
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