SOLUBLE T-CELL ANTIGEN RECEPTORS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 469

patients with advanced bladder cancer [55]. In our ongoing
clinical studies, patient is selected based on serologic
HLA-A2 testing followed by assessment of tumor biopsy
specimens for immunohistochemically identifiable p53
(aa 264-272)/HLA-A 0201 complex. Multicycle ALT-
801/chemotherapy treatment is administered under an out-
patient setting with the initial objective of determining the
safety profile and tolerability during a dose escalation stage
of the studies. Upon establishment of the MTD regimen,
patientenrollmentwillbeexpanded based on the two-stage
study design to further evaluate the safety of the ALT-
801/chemotherapy regimens and assess tumor objective
response and clinical benefit rates and patient survival rates.
Phase II trials are also planned that will further evaluate
the efficacy and safety of ALT-801 treatment in an
expanded population of patients with prostate and head
and neck cancers as ALT-801 monotherapy showed some
evidence of clinical benefit in these indications in the
completed Phase I trial. The safety profile and efficacy
results of ALT-801/chemotherapy being defined in the
current Phase Ib/II studies will be used in designing these
and future controlled studies of ALT-801-based treatment
regimens for cancer indications. In addition, successful
demonstration of clinical activity of the scTCR-mediated
tumor-targeting approach will allow clinical advancement
of ALT-802 for NSCLC and other tumor antigen-specific
STAR fusion proteins (i.e., anti-MAGE-A3, gp100, or
MART-1) currently in research and preclinical studies in
our laboratory.

at the expense of memory cell formation, and supporting
activation-induced cell death (AICD) that leads to the
elimination of stimulated T cells and induction of T-cell
tolerance [56]. In contrast, IL-15 inhibits IL-2-induced
AICD and supports long-lasting CD8 þ T-cell memory
and effector responses [56], suggesting that IL-15 may
have significant advantages over IL-2 for the treatment of
cancer [57]. Indeed, a recent NCI review listed IL-15 as the
most promising product candidate among 12 immuno-
therapy drugs that could potentially cure cancer [58].
We have generated and characterized a number of
different soluble scTCR fusion proteins to IL-15 and to
novel IL-15 super agonists and antagonists. We have
shown that scTCR/IL-15 fusion molecules containing an
N72D mutation in the IL-15 domain exhibit a three-to-
fourfold increase in IL-15 biological activity, whereas
those with an IL-15 D8N mutation exhibit little or no
activity [59]. While IL-15 superagonist-based fusion pro-
teins could serve as targeted immunotherapeutics for
cancer and infectious diseases, an IL-15 antagonist capable
of inhibiting IL-15 responsive cells at the disease site may
have therapeutic potential in treating allograft rejection
and inflammatory autoimmune diseases, particularly if
memory CD8 þ T cells play a role in disease pathology
[56]. A nontargeted IL-15 mutant/Fc g 2a antagonist protein
has already been shown to be effective at inhibiting islet
and cardiac allograft rejection and preventing development
and progression of arthritis in experimental animal models
[60,61].
IL-15 can be effectively transpresented to IL-2R bg -
bearing immune cells by the IL-15R a chain on APCs or by
soluble IL-15R a domains [56,62,63]. As a result, we have
generated scTCR/IL-15R a fusion proteins capable of
forming functional soluble multimeric complexes with
the scTCR/IL-15 fusions (Figure 31.1). These fusion pro-
tein complexes have several advantages as targeted immu-
notherapeutics by providing improved and more
biologically relevant functional activity and an increased
serum half-life compared to IL-15 alone as well as multi-
valent-targeting activity directed to the TCR-specific
tumor antigen [22,59]. In fact, we have found that the
IL-15 and soluble IL-15R a domains could be used as a
protein scaffold much like the antibody constant domains
to create various novel molecules, including bivalent com-
plexes containing the same scTCR domains, bispecific
complexes containing different scTCRs, scAb, and other
binding domains, and heterodimeric complexes of TCR a
and b chains [22]. As a further refinement, we have also
generated scTCR-targeted IL-15/IL-15R a complexes
comprising IgG Fc domains. Such complexes are capable
of binding and activating both IL-15R bg C-bearing and
Fc receptor-bearing human immune cells and mediating
both cytokine-induced and ADCC activity against target
cells. We have found that administration of mice or

31.6 ALTERNATIVES/VARIANTS
OF THIS APPROACH

31.6.1 Development of Novel TCR-Targeted IL-15
Superagonist Fusions

The success of utilizing scTCR domains in targeted immu-
notherapeutic approaches has prompted us to explore
fusions with other cytokines and effector molecules includ-
ing TNF- a , GM-CSF, and IL-15. Particularly, there has
been considerable interest recently in the potential of the
immunostimulatory cytokine IL-15 to supplant IL-2 as a
therapeutic agent for cancer. This is due to the distinct and
contrasting roles of IL-15 and IL-2 in the life cycle
of effector lymphocytes [56]. Both IL-2 and IL-15 are
capable of stimulating immune cells via interactions
with shared signaling receptor components, IL-2R b and
IL-2R g C, but unique a chain subunits (IL-2R a and
IL-15R a ) are required for high-affinity binding of each
cytokine to the IL-2R bg complex. The effectiveness of
IL-2 as an anticancer therapeutic has been questioned
based on its pivotal role in the maintenance of CD4 þ
CD25 þ T-regulatory cells, promoting CTL differentiation