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Vol. 26, Issue 4, 318-323, April 1998
Faculty of Pharmaceutical Sciences, Kyushu University (S.T., H.M.,
Y.S.), and
Department of Pharmacy, University of Tokyo Hospital,
Faculty of Medicine, University of Tokyo (Ka.Y., Ko.Y., T.I.)
To quantitatively evaluate drug-drug interactions involving
metabolic processes in the liver, we attempted to predict the increasing ratio of the plasma concentration of midazolam (MDZ) in the
presence of cimetidine (CIM) or nizatidine (NZD) in rats. Under
steady-state conditions for the plasma concentration of CIM or NZD, MDZ
was administered through the portal vein. The AUC of MDZ in the
presence of CIM was 2.5-fold higher than that in the absence of CIM.
There was no effect of NZD on the AUC of MDZ. The liver/plasma
concentration ratios for CIM and NZD were 4.0 and 2.7, respectively.
The estimated liver unbound concentration (CH,f)/plasma unbound concentration
(Cp,f) ratios for CIM and NZD were 1.9 and 2.4, respectively, suggesting concentrative hepatic accumulation of
both drugs. The oxidative metabolism of MDZ in rat liver microsomes was
competitively inhibited by CIM or NZD, and the
Ki values of CIM and NZD were 110 and
2600 µM, respectively. Based on these data obtained in
vivo and in vitro, the increasing ratios for MDZ in
the presence of CIM or NZD were predicted using the equations
Rp = 1 + Cp,f/Ki and
RH = 1 + CH,f/Ki.
The observed increasing ratios in the presence of CIM were very close
to RH, compared with
Rp. However,
Cp,f and
CH,f were much less than
Ki and there was no difference between
Rp and RH
in the presence of NZD. Consequently,
Cp,f and
CH,f were greater than or equal to
Ki and Cp,f
was not equal to CH,f, as in the presence
of CIM, and it was indicated that CH,f
was more suitable for quantitatively predicting the drug-drug
interactions than was Cp,f.
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