Oral administration of malonyl-sildenafil suppresses
proliferation in the colon epithelium.
The pharmacokinetic data support the idea that malonyl-sildenafil has
reduced permeability and does not enter the circulation following oral
administration in mice. It was therefore important to determine whether
malonyl-sildenafil could enter the colon epithelium to suppress
proliferation, which is thought to be the central effect of PDE5i that
mediates colon cancer prevention (Browning, 2019; Islam et al. ,
2018). Mice were maintained on malonyl-sildenafil (9 mg/kg) in the
drinking water for up to 8 days, and there was no difference in body
weight compared to those on sildenafil or water alone (Fig. 6A). The
initial drop in weight in the first couple of days most likely reflected
a distaste for the compound since they initially drank less than the
control groups. Mice were active and exhibited no observed deleterious
effects during the time studied. Histological analysis of the colon
mucosa after 8 days consuming malonyl-sildenafil revealed no obvious
epithelial damage, edema, or leukocyte infiltration (Fig. 6B).
Malonyl-sildenafil was not detected in the plasma of mice drinking
malonyl-sildenafil for 8 days, but low levels of sildenafil (3.3 nM)
were observed in the plasma of mice drinking sildenafil (Fig. 6C). To
determine the effect of orally administered malonyl-sildenafil on
proliferation, the epithelium from the proximal half of the colons was
isolated and subjected to flow cytometry to detect the proliferative
marker Ki67 (Fig. 7A, B). This showed a median of 23%
Ki67+ cells in the control animals on drinking water
alone, but this was significantly reduced to 11% and 7% by sildenafil
and malonyl-sildenafil treatment (respectively). Sections of the distal
colon were also stained for PCNA, which showed the expected
proliferative compartment in the lower third of the crypts (Fig. 6C).
Both sildenafil and malonyl-sildenafil reduced the number of
PCNA+ cells/crypt by about 30% (Fig. 6D). Taken
together these results demonstrate that orally administered
malonyl-sildenafil and sildenafil were equally effective at suppressing
proliferation in the colon epithelium of mice.