Dipeptidyl Peptidases: Substrates and Therapeutic Targeting in Human Health and Disease - Proteinases as Drug Targets - page 25

1.7.1.1 Targeting DP8 and DP9
Selective inhibition of DP8/9 results in attenuation of T-cell proliferation and
release of the cytokine IL-2. 11 In human T-cells, the synthesis of DNA and thus
cell proliferation can be suppressed upon treatment of cells with the DP8/9
selective inhibitor, UAMC00132. 134 Similar evidence provided by Reinhold
et al. demonstrates that selective inhibition of DP8/9 activity with UAMC00132
results in suppression of DNA synthesis in mitogen-stimulated splenocytes
isolated from both wild-type and DP4 knockout mice. 239 Recently, DP8/9 have
been implicated in the allergic response of the lung in rat models of experimental
induced asthma. 240 Asthma is a well-known chronic inflammatory disease of the
airways characterized by airway obstruction and resulting in breathing di-
culties, typically wheezing, coughing, and breathlessness. Thus, future studies
investigating short-term suppression of the immune response during asthma
attacks via selective DP8/9 inhibition may prove to be of interest. In research
performed by our own group, the expression of DP8 at the transcript level was
found to be significantly upregulated in C57BL/6J mice with DSS-induced
colitis compared to untreated controls, 241 thus providing additional evidence for
a role of DP8 in acute inflammatory responses.
Controversy has surrounded the potential safety in targeting DP8/9 activity
making their suitability as therapeutic targets questionable. Using the DP8/9
selective inhibitor, UAMC00132, over a 2-week period at doses of 10, 30, and
100 mg/kg -1 , rats reportedly developed toxic effects including alopecia,
thrombocytopenia, reticulocytopenia, enlarged spleen, inflammatory cell infil-
tration of multiple organs, including the gastrointestinal tract, and mortality. 11
In contrast a rebuttal study by Burkey et al. using Vildagliptin s at high doses
of 1500 mg/kg/day for 12 weeks in mice and 900 mg/kg for 12 weeks in rats
observed no toxicity or mortality of animals. 256 Vildagliptin s was designed to
selectively inhibit DP4, but a modest level of DP8/9 inhibition remains at these
high doses. Wu et al. used an alternative DP8/9 selective inhibitor, 1G244,
displaying a 15- and eightfold more potent inhibition against DP8/9 than the
inhibitor UAMC00132, to assess toxicity, reporting that although toxic effects
were not observed, some mild symptoms were identified. 257 One of the more
concerning mild symptoms observed in animals receiving higher doses of 10 or
30 mg kg -1 day -1 was the development of severe cyanosis which resulted in
animals being killed on days 4 and 5 of the 2-week study. 257 Cyanosis also
developed at the lower dose of 1 mg kg -1 day -1 . Development of cyanosis
suggests that the inhibitor 1G244 may be causing a hypoxic effect in animals,
allowing us to speculate on the involvement of DP8/9 in this process. Without
further evidence from a more complete study it cannot be definitively concluded
that 1G244, and thus the inhibition of DP8/9, does not cause toxicity. Although
progress has been made towards the development of potent and selective
inhibitors of DP8 and DP9, 144,258-269 an inhibitor that can selectively differ-
entiate between these two is still lacking. Such an inhibitor would help to
further elucidate the enzyme, if any, responsible for the apparent adverse affects
following DP8/9 inhibition.