SOLUBLE T-CELL ANTIGEN RECEPTORS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 470

cynomologous monkeys with tetrameric scTCR/IL-15:
scTCR/IL-15R a /Fc complexes resulted in significant induc-
tion of NK and CD8 þ T-cell proliferation and immune
responsiveness. The p53-specific scTCR/IL-15:scTCR/IL-
15R a /Fc complexes were also capable of mediating
antitumor activity against subcutaneous p53 þ human tumors
in the A375 melanoma xenograft model described earlier.
Together these results indicate that multivalent scTCR tar-
geted IL-15 molecules could represent highly potent and
novel immunotherapeutics for treating cancer and viral
infections.

cell-surface bound ALT-801 (Figure 31.4C) [23]. Based
on these results, we have initiated a Phase I clinical trial
that will employ ALT-801 in combination with NK cell-
based donor lymphocyte infusion therapy in patients with
acute myeloid leukemia (AML). It has previously been
reported that adoptive transfer of haploidentical NK cells
together with IL-2 therapy following nonmyeloablative
immunosuppressive conditioning resulted in complete hem-
atologic remission in 5 of 19 patients with poor-prognosis
AML [69]. The current study is designed to evaluate the
feasibility of using ALT-801 as a substitute for IL-2 for the
activation of haploidentical NK cells prior to adoptive
transfer and as a targeted immunostimulatory therapy post-
cell transfer. The outcome of this study may provide an
assessment of the safety and antitumor efficacy of ALT-801-
targeted cellular therapy compared to nontargeted
approaches. Similar strategies using adoptively transferred
autologous T cells in combination with STAR-cytokine or
scTCR/anti-CD3 scAb fusion protein are also being investi-
gated and may provide more practical clinical applications
for treating cancer or virally infected patients than geneti-
cally engineered lymphocyte approaches.

31.6.2 Cell-Based Therapies

Adoptive T-cell therapy employing autologous engineered
T cells generated with gene transfer techniques using
tumor antigen-specific TCR genes is currently a focus
of intense interest in the field of cancer treatment and
has been the subject of a number of recent reviews [64,65].
Generally, these approaches employ introduction of a / b
TCR genes into the patient's peripheral blood lymphocytes
via viral expression vector systems. Following expansion
and in vitro functional characterization, the genetically
modified autologous lymphocytes are adoptively trans-
ferred into lymphodeleted patients, who are subsequently
treated IL-2 to promoter expansion of the transferred cells.
Strategies to improve stable expression, functional pairing,
and signaling of the transferred TCR genes as well as
affinity improvement for antigen specific recognition by
the engineered lymphocytes are currently being investi-
gated. In early stage clinical trials, adoptive transfer of
autologous T-cells expressing recombinant TCRs specific
to MART-1, gp100, NY-ESO-1, and carcinoembryonic
antigen (CEA) has been reported to provide objective
antitumor response rates of 12-66% in patients with
metastatic melanoma, colorectal cancer, and synovial
cell sarcoma [36,66-68]. Additionally, destruction of
normal melanocytes in the skin, eye, and ear following
MART-1- and gp-100-specific T-cell transfer and colitis
following CEA-specific T-cell transfer suggest that these
therapies

31.7 CHALLENGES

STAR fusion-base therapies represent personalized medi-
cine in the eligible patients who express the appropriate
HLA allele and display the peptide antigen on the diseased
tissue recognized by the scTCR domain. As a result, the
patient population likely to benefit from such treatments
represents a subset of the total population afflicted with the
disease. Consequently, selection of broad-based disease-
relevant pMHC targets is an important consideration while
employing this strategy. For our lead anticancer immuno-
therapeutic, we are targeting tumor cells presenting a pep-
tide epitope (aa264-272) of p53 in the context of HLA-
A 0201, the most common MHC class I allele. The aa264-
272 region of p53 is rarely mutated in cancer cells, its cell-
surface display likely will serve as a more broadly presented
antigenic target for tumors with abnormal p53 expression
than peptide epitopes bearing specific p53 mutations, the
most frequent of which accounts for only 5% of possible p53
variants [70]. As indicated earlier, p53 overexpression is
commonly observed in

can exhibit potent immune cell-mediated

reactivity against normal tissues expressing the target

antigen [36,68].
We are exploring similar T cell based therapeutic strate-
gies employing our portfolio of scTCR domains linked to
lymphocyte signaling domains as novel chimeric receptors
reactive to tumor and viral antigen targets. Additionally, we
have been interested in evaluating therapies combining
soluble STAR fusion proteins with adoptively transferred
lymphocytes. In preclinical studies, we have shown in that
ALT-801 can stably bind to IL-2R-bearing lymphocytes.
Following adoptive transfer into p53 þ tumor-bearing mice,
the ALT-801-treated lymphocytes preferentially accumulate
at

50% of tumors of different origins
[30]. Consistent with these findings, we have found that 40%
of the patients with different metastatic malignancies
screened in our Phase I/II clinical studies (n

200) were
HLA-A2 positive and had tumors that presented the p53
(aa264-272)/HLA-A 0201 complex [51]. These values com-
pare favorably to the 22% tumor positivity rate of HER-2
gene amplification/protein overexpression observed in
breast cancer patients as a screening criterium for anti-
HER-2 mAb-based therapies [71].

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the tumors site, suggesting tumor targeting via