SOLUBLE T-CELL ANTIGEN RECEPTORS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 471

Another important consideration for TCR-based thera-
pies is the level of antigenic pMHC complexes displayed on
the diseased cells. Antigen presentation is a complex process
that is influenced by the level of endogenous or exogenous
protein present, appropriate degradation of these proteins by
the proteasome machinery, the ability of the degraded
peptides to bind the groove of the particular MHC molecules
and the successful transport and stability of the pMHC
complex on the cell surface. Although we have found that
the p53 (aa264-272)/HLA-A 0201 complex to be displayed
at up to 2000 complexes per cell on the surface of different
human tumor cells [15], other researchers using pMHC-
specific reagents or T-cell lines have shown only moderate to
very low expression of tumor cell presentation of HLA-
A 0201-bound peptides derived from well-characterized
tumor-associated antigens including gp100, MART-
1/melan-A, NY-ESO-1/LAGE-1, and telomerase reverse
transcriptase [72-74]. For example, NY-ESO-1-positive
tumor cells were found to present only 10-50 NY-ESO-1
(aa157-165)/HLA-A 0201 complexes per cell, a level which
may be insufficient for a targeted protein-based therapeutic
approach [73]. Additionally, intracellular expression levels
of the tumor-associated antigens may not directly correlate
with the level of the derived peptide/HLA-A 0201 com-
plexes displayed on the cell surface or immune cell
reactivity to these complexes [72,75]. These findings clearly
indicate that detail characterization of the target pMHC
levels and efficacy analysis against different antigen-posi-
tive target cells are required during the preclinical assess-
ment of any given TCR-based therapy. Moreover,
concurrent development of companion diagnostic methods
to identify subjects with the appropriate disease-related
pMHC presentation is likely an important component in
the clinical evaluation of such an approach.

in such studies will facilitate clinical development of other
p53-specific scTCR fusion molecules, including ALT-802,
in cancer indications and establish the STAR technology
platform as a next-generation targeted immunotherapeutic
approach for treatment of patients with cancer and viral
infections. As indicated, we are currently developing a
portfolio of STAR fusion proteins reactive to tumor and
viral antigens as follow-up clinical candidates to the
p53-specific molecules.
Additionally, we are evaluating the targeted activity of
novel immune effector domain fusion proteins against tumor
and virally infected cells in various preclinical models. Of
these proteins, the IL-15:IL-15R a -Ig Fc complex equipped
with scTCRs or scFv antibody domains shows great poten-
tial as a targeted immunotherapeutic fusion capable of
promoting cytokine-induced NK cell and CD8 þ T-cell
responses as well as ADCC and complement activation
mediated by the Fc domain. Moreover, IL-15 has been
shown to augment ADCC activity of NK cells, suggesting
synergy between these effector domains [76]. Such multi-
valent antigen-specific fusion proteins are actively being
investigated in our laboratory to assess whether this
approach is capable of further improving the therapeutic
activity of clinically proven antibody molecules.

ACKNOWLEDGMENTS

The work to develop the STAR technology was supported in
part by grants from the National Institute of Health
(CA088615, CA105816, AI056782, CA097550,
CA117211, A1083130, CA139810, and AI093218), the
Food and Drug Administration (FDR03452) and the Bill
&Melinda Gates Foundation. The author thanks Dr. Hing C.
Wong for helpful suggestions and critical review of the
manuscript and Drs. Jack Egan, Jinghai Wen, and Wenxin
Xu of Altor BioScience

31.8 CONCLUSIONS AND FUTURE
PERSPECTIVES

for allowing inclusion of

unpublished data.

ALT-801, a p53-specific scTCR-IL-2 fusion protein, repre-
sents the first-in-class soluble TCR-based protein therapeu-
tic to enter clinical studies. As described in this review,
rigorous preclinical studies established the potent-targeted
antitumor activity, mechanism-of-action and favorable phar-
macokinetic and safety profiles of this molecule. Initial
clinical results of ALT-801 in patents with metastatic malig-
nancies are encouraging. However, further evaluation in
Phase II/III clinical studies is required to establish proof-
of-concept of this approach in patients. We are conducting
clinical studies of ALT-801 in different cancer indications,
where p53 overexpression in thought to play a role in disease
progression, and in combination with different chemo-
therapy regimens, which may augment antitumor activity
of ALT-801. Positive results establishing ALT-801 efficacy

REFERENCES

1. van der Merwe PA, Davis SJ. (2003) Molecular interactions
mediating T cell antigen recognition. Annu. Rev. Immunol. 21,
659-684.
2. Davis MM, Bjorkman PJ. (1988) T-cell antigen receptor genes
and T-cell recognition. Nature 334, 395-402.
3. Admon A, Barnea E, Ziv T. (2003) Tumor antigens and
proteomics from the point of view of the major histo-
compatibility complex peptides. Mol. Cell. Proteomics 2,
388-398.
4. Casiano CA, Mediavilla-Varela M, Tan EM. (2006) Tumor-
associated antigen arrays for the serological diagnosis of
cancer. Mol. Cell. Proteomics 5, 1745-1759.