CLASSIC IMMUNOTOXINS WITH PLANT OR MICROBIAL TOXINS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 269

18.4 CLINICAL UPDATE OF IMMUNOTOXIN
TRIALS

trials, either alone or in combination with other treatment for
treatment of various cancers including B-cell NHL, T-cell
NHL, melanoma, advanced breast cancer, pancreatic cancer,
ovarian cancer, and AML (acute myeloid leukemia).
BL22 (RFB4(dsFv)-PE38) consists of a disulfide-stabi-
lized anti-CD22 Fv fused to PE38. When given at intra-
venous doses of 3-50 m g/kg every other day for three doses
every month for up to 14 cycles to patients with chemo-
therapy-resistant B-cell malignancies, there were 77% over-
all response rate (61% CR and 16% PR) in hairy cell
leukemia (HCL) [133]. In a Phase II for HCL, BL22
treatment at doses of 40 m g/kg every other day for three
doses every month produced 72% overall response rate (47%
CR and 25% PR) [134]. HA22 is an affinity optimized, more
active variant of BL22 and is currently in a clinical trial for
refractory B-cell malignancies. The affinity of HA22 to
CD22 was improved 14-fold by phage display affinity
maturation [135]. In a Phase I clinical trial for HCL,
HA22 was administered at intravenous doses of 5-50 m g/kg
every other day for three doses every month up to 10 cycles.
The overall response rate was 79% including 12 CR (43%)
of 28 HCL patients. No dose-limiting toxicity was
observed after completion of expansion at

Among over a thousand immmunotoxins, only about
40 immunotoxins have been tested in clinical trials
(Table 18.4). Most of immunotoxins have failed to reach
clinical testing because of poor in vivo efficacy. Immunotox-
ins with high response rates in clinical studies are highly
active in cell proliferation inhibition assays using target cell
lines. Typical half maximal inhibitory concentrations (IC 50 )
of immunotoxins in the assays are in the picomolar range. In
contrast, many immunotoxins discontinued at the preclinical
stages have poor cytotoxic activity in cell proliferation inhi-
bition assays (the nanomolar range of IC 50 ) and poor
antitumor activity in animal tumor models. Tumor regression
is typically observed at high doses

1mg/kg/day. These dose
levels are at least twofold higher than the highest MTD
(0.5mg/kg/day) observed in immunotoxin clinical trials.
High response rates have been observed in clinical trials
of immunotoxins for hematologic malignancies. Target
antigens for hematologic malignancies are CD3, CD5,
CD19, CD22, CD25, and IL2R. Expression of these antigens
is restricted on hematologic cells. This allows immunotoxins
to treat patients at sufficient doses producing clinical
responses without severe toxicities. However, some target
antigens such as EGF receptor, Her2, Lewis Y, transferrin
receptor (TfR), and a 55 kDa antigen expressed on breast
carcinomas causes dose-limiting toxicities because of the
presence of target antigens on normal tissues.
DAB 389 IL2 (Ontak 1 , denileukin diftitox) is the first
FDA-approved immunotoxin for the treatment of persistent
or recurrent CTCL. In the first Phase III trial, DAB 389 IL2
was administered at 9 or 18 m g/kg/day by intravenous
infusion over 30-60 min for five consecutive days every 3
weeks for eight cycles. Thirty percentage of the 71 patients
with CTCL experienced an objective response (10% com-
plete remission [CR] and 20% partial remission [PR]) [129].
In the other Phase III placebo-controlled trial of DAB 389 IL2,
10%CR and 34% PR were observed in CTCL patients [130].
In other trials of DAB 386 IL2 for expansion of indications,
DAB 386 IL2 produced transient T-cell depletion in mela-
noma patients, causing regression of melanoma metastases
in 4 out of 16 patients, suggesting that transient T-cell
depletion may disrupt the homeostatic control of cognate
immunity and allow for the expansion of effector T cells
with specificity against neoplastic cells during homeostatic
T-cell proliferation [131]. In a Phase II trial for T-cell NHL
(peripheral T-cell lymphoma, anaplastic large cell lym-
phoma, angioimmunoblastic T-cell lymphoma, Sezary syn-
drome, and NK/T-cell lymphoma), six CRs and seven PRs
were observed in 27 patients (overall response rate 48.1%).
Interestingly objective responses were achieved in 8 of 13
patients (61.5%) with CD25

>

the highest

dose (50 m g/kg 3) cohort (12 patients) [136].

LMB2 (anti-Tac(sFv)-PE38) is composed of an anti-
CD25 sFv fused to PE38. LMB2 was given to 35 patients
with lymphomas at intravenous doses of 2-63 m g/kg every
other day for three doses. LMB2 therapy produced one
durable CR and 7 PR (overall response rate: 23%) [117].
LMB2 was administered with MART-1/gp-100 vaccine to
patients with metastatic melanoma in an effort to augment
antimelanoma immunity via T reg depletion. LMB2 admin-
istration at dose of 30 m g/kg every other day for three doses
transiently reduced 79.1% of CD25 positive T cell [137].
A-dmDT390-bisFv(UCHT1) contains diphtheria toxin
(DT), preceded by an alanine residue (A), truncated at amino
acid residue 390, doubly mutated (dm) to remove glycosyl-
ation sites within the toxin that reduce bioactivity when
expressed in P. pastoris and eukaryotic cells [99], and
two tandem sFv molecules derived from UCHT1 parental
antibody. A-dmDT390-bisFv(UCHT1) is being tested in a
Phase I clinical trial [138]. After eight infusions of 2.5 or
5.0 m g/kg over a 4-day period blood T cells are depleted by
three logs. CD4 and CD8 T cell numbers return to pretreat-
ment levels by day 20 but the na ıve subsets are markedly
depressed for several months. One course of A-dmDT390-
bisFv(UCHT1) treatment produced durable remissions last-
ing over two years. Of eight eligible patients with CTCL,
two patients had CR (25%) and three patients had PR
(37.5%). Overall response rate was 62.5%.
Transferrin-CRM107 (TransMID) is a chemical conju-
gate of transferrin and a mutant DT (CRM7) lacking recep-
tor-binding function [1] for the treatment of malignant brain
tumors. In a Phase I trial, 18 patients with recurrent

tumors (4 CRs and 4 PRs)
[132]. DAB 389 IL2 is currently being evaluated in clinical

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