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Development of Novel Immunotherapeutics 173
plasmid by various vaccination strategies, there seems to be an intrinsic limitation
to overall magnitude of immune response. More recent evidence suggested
that the reduced transcriptional competency of transfected APC might be the key
limiting factor in this regard. This is also supported by the finding that mRNAbased
vectors may be able to circumvent this bottleneck (15). Consequently,
simply increasing the transfection of resident cells will not necessarily result in a
substantial magnification of the immune response.
One of the methodologies to address the overall poor immunogenicity of
plasmids was based on the prime-boost approaches. This methodology was aimed
to build on the quality of the response elicited by plasmids but complement them
with other vectors that are capable to provide a more optimal antigen exposure.
Aside microbial vectors (recombinant viruses or microbes), there are very limited
options on what agents can be used as boosters: recombinant proteins, polypeptides,
cells, and tumor cell lysates. Peptides represent a unique opportunity in
light of targeted intra–lymph node delivery since their known suboptimal pharmacokinetic
profile when delivered via more conventional routes. Direct peptide
injection into lymph nodes, as shown by preclinical studies, achieves a substantial
loading of resident APC and, as a result, robust immunity (16). Consequently,
plasmid priming followed by peptide boost resulted in even greater amplification
of immunity, retaining the profile of immune response imprinted by plasmid
priming and dominated by CD62L – CD44 hi CD27 hi T cells capable to produce
IFN-g, TNF-a, MIP1a, and RANTES, externalize CD107a, and produce granzyme
B upon antigenic challenge (17). This approach, however, achieves
expansion of immune responses only against defined epitopes (one epitope per
boosting peptide), but not all epitopes that are encompassed by plasmid inserts. To
be effective, this approach needs to target epitopes expressed on a majority of
tumor cells, and the methodology elicits “epitope-spreading” associated with
progressive broadening of immunity against multiple tumor epitopes and antigens.
Currently, there are no reliable experimental means to validate such epitopes in
humans; to diminish the risk associated with monoepitope or monovalent
approaches, we pursued multicomponent, multivalent approaches—flexible
enough to allow mixing and matching of the components fitting the patient’s
tumor antigen expression profile. On the basis of in depth understanding of the
MOA, this is consistent with the principle of personalized or stratified medicine
allowing to treat the patients who have the higher likelihood of response. Finally,
an expanded array of assays is needed to explore in a comprehensive fashion the
pharmacological response and improve on the likelihood of correlating aspects of
the biological response with clinical outcome, as well as establish a cause-effect
relationship between the investigational drug and clinical effect. This latter aspect
is key to directing subsequent development of cancer vaccines in addition to
providing decisional flexibility based on solid data sets.
Overall, this translated into two optimized, multicomponent, investigational
drugs that are either in clinical trials (18,19) or in the last preclinical
development stages (Fig. 14). The peptide analogues used as boosting agents