Malonyl-sildenafil does not enter the circulation following oral
administration in mice.
In order to determine the extent to which malonyl-sildenafil can enter
the circulation, we developed an LC-MS/MS procedure to measure the
compound in plasma (Fig. 3). The ion spectrum used for sildenafil is
based upon a previous study (Drüeke et al. , 2019), and several
common peaks were generated by malonyl-sildenafil including m/z at 283,
311 and 377. However, additional peaks were observed that are consistent
with loss of the carboxylic group (m/z = 503), and loss of the whole
malonyl-group to generate desmethyl-sildenafil (m/z = 461). One hour
after oral administration of malonyl-sildenafil to mice at
concentrations up to 36 mg/kg did not result in detectable levels in the
plasma (Fig 4A). This contrasted with sildenafil that was detected in
plasma at 250 nM and 3.5 µM after administering 9 and 36 mg/kg
(respectively). Notably, sildenafil was not detected in the feces
collected between 2-4 hours after oral administration, but
malonyl-sildenafil was detected at high levels ranging from 90 µg/g to
244 µg/g after administering 9 and 36 mg/kg respectively (Fig. 4B). The
decision to measure malonyl-sildenafil one hour following oral
administration was based upon established pharmacokinetics for
sildenafil (Nichols et al. , 2002; Walker et al. , 1999).
The possibility that the reduced membrane permeability of
malonyl-sildenafil could cause delayed entry into the circulation was
tested by performing a time course using 9 mg/kg (Fig. 4C). Apart from a
very small (11 nM) peak at 1 hour, these experiments revealed that there
was no delayed peak, and malonyl-sildenafil was not detected in the
plasma at any other time during the 24-hour period examined. Notably,
malonyl-sildenafil continued to be excreted at high levels in the feces
8 hours after oral administration (Fig. 4D).
Consuming PDE5i together with soluble guanylate cyclase stimulators such
as riociguat, or nitrate activators such as isosorbide mononitrate
(ISMN) and nitroglycerin is contraindicated due to the risk of severe
hypotension. Malonyl-sildenafil was not detected at high levels in the
plasma compared to sildenafil, but bioactive metabolites could have been
created that could enter the circulation undetected. To investigate this
possibility, mice were treated with ISMN (300 mg/kg i.p.) alone, or
together with orally administered sildenafil or malonyl-sildenafil (10
mg/kg). Arterial blood pressure was then measured over time using remote
telemetry (Fig. 5). ISMN alone or with malonyl-sildenafil slightly
increased blood pressure from a baseline of around 100 mmHg to 126 mmHg
and 117 mmHg (respectively). This was most likely due to stress caused
by the procedure. As expected, co-administration of ISMN and sildenafil
caused a rapid drop in blood pressure to 68 mmHg that returned to close
to the baseline over the following 30 minutes.