crc press - E-Lib FK UWKS

Hydrophobic Membrane Translocating Sequence Peptides 131
The kFGF MTS, penetratin, and HIV-1 Tat peptides all fulfill these criteria and
are currently regarded as the leading translocating peptide vectors. Whether prolinerich
peptides will also meet all of these requirements remains to be determined,
although results obtained to date are promising. Until a definitive study is performed
in which the ability of different translocating peptides to transport the same cargo
into cells is directly compared, the choice of peptide will largely be influenced by
the types of uses described in the literature.
The ultimate goal of our translocating peptide research is to design biologically
active therapeutic compounds. For this, potentially three different segments must be
included in a single compound: a targeting module, a penetrating module, and an
effector module. The role of the targeting module would be to direct the attached
penetrating and biologically active modules to specific cell types in vivo, e.g., cancer
cells and virus-infected cells. One of the defining characterisitics of cancer cells is
the overexpression of specific cell surface proteins 166-170 and this feature may be
exploited in the design of anticancer agents.
Effective antiviral agents may also be assembled where a biologically active
antiviral drug such as a reverse transcriptase inhibitor can be linked to a cell-permeant
peptide vector and targeted to infected cells. Virus-infected cells often express virusspecific
proteins on the cell surface during replication; therefore, identification of a
suitable target may be simplified. For example, human cytomegalovirus chemokine
receptor US28 is expressed at the cell surface 171 and a number of HIV surface
proteins are expressed prior to virion formation and budding. 172 Furthermore, this
approach could be applied to development of a compound aimed at decreasing the
multidrug resistance (MDR) exhibited by cancer cells.
Many cancer types, including leukemia, multiple myeloma, lymphomas, and a
variety of solid tumors, overexpress the cell surface protein P-glycoprotein (P-gp)
that transports anticancer drugs out of cells before cytotoxic effects occur. 166 The
administration of drugs capable of reversing this MDR is currently limited because
of the toxic effect on normal tissues. The assembly of a molecule that can target
and bind to the overexpressed P-gp protein, penetrate through the cell membrane,
and deliver a chemotherapeutic agent to the cancer cell resulting in cell death is
highly desirable.
A major concern raised by Derossi et al. 9 is that the high efficiency of internalization
exhibited by translocating peptides in vitro could have a detrimental effect
on in vivo delivery because the peptide may be captured by cells at the delivery
point and not reach target cells. Attachment of a targeting module addresses this
concern to a certain extent. Ideally, the penetrating module would be completely
masked by the targeting module and unavailable for nonspecific membrane interaction.
Upon binding to the target receptor the target module would undergo a conformational
change and expose the penetrating module to the lipid bilayer, thus
allowing translocation to commence.
In conclusion, the production of drugs and diagnostic tools involves not only
the design of the empirical three-dimensional conformation of the biologically active
component but also the rational design of an effective intracellular delivery system
for subcellular activity. Hydrophobic MTS peptides have already proven their worth
as peptide vectors through the delivery of peptides and proteins to the intracellular