Propentofylline (PPT, 3-methyl-1-(5-oxohexyl)-7-propyl-xanthine)
has been reported to be beneficial in the treatment of
both vascular dementia and Alzheimer type dementia (1-4). PPT
increases the solubility of lipids by substituting a methyl group
for a propyl one in position 7 of the purine backbone of pentoxifylline.
The pharmacological effects of PPT may be observed in stimulation of the nerve growth factor, increased cerebral blood flow and inhibition
of adenosine uptake. PPT also enhances extracellular adenosine concentrations and
decreases extracellular levels of glutamate in vivo during ischemia (5,6).
To date, despite the known pharmacological effects, few clinical pharmacokinetic
or metabolism studies of PPT have been reported in the literature. The short
half-life of PPT at the terminal elimination phase and poor bioavailability, after oral
administration to rabbits, suggest that this drug undergoes extensive first-pass metabolism
in liver. PPT is metabolised in vitro to the racemic compound –
(±)-1-(5-hydroxyhexyl)-3-methyl-7-propyl-xanthine (HOPPT) (7-10).
Studies have shown that the racemic mixture and the steroisomers of HOPPT
demonstrate biological activity. Hydroxy-metabolites of PPT inhibited [3H]nitrobenzylthioinosyne
binding in rat brains with a similar affinity to propentofylline,
and also inhibited [3H]adenosine uptake by es transport as effectively as propentofylline.
Since inhibition of adenosine transport appears to be important for the
neuroprotective effects of PPT, the hydroxy-metabolites may also provide neuroprotection.
As steroisomers of HOPPT are not commercially available, we had to
produce them for pharmacological and pharmacokinetic studies.
In this paper, we present the results of stereoselective biotransformation of PPT
to HOPPT when catalysed by whole cells of baker’s and a few strains of wine yeast in
both water and organic solvents. Alcohol dehydrogenases (YADH’s) in yeast
(Saccharomyces cerevisiae) play an important role in the reduction of carbonyl functions
in aldehydes and ketones (11). YADH has very narrow substrate specificity and
generally accepts only aldehydes and methyl ketones. It is therefore of only limited
use in the preparation of the chiral secondary alcohols.
Microbiological reduction with yeast was carried out under non–fermenting conditions.
The stereoselectivity of this biotransformation process was determined using
the HPLC technique with a chiral column. It was established that different yeast
strains favoured the biotransformation of PPT into different enantiomers.