CLINICAL PHARMACOLOGY
Mechanism of Action
Ursodiol, a naturally occurring
hydrophilic bile acid, derived from cholesterol, is present as a minor fraction
of the total human bile acid pool. Oral administration of ursodiol increases
this fraction in a dose related manner, to become the major biliary acid,
replacing/displacing toxic concentrations of endogenous hydrophobic bile acids
that tend to accumulate in cholestatic liver disease. In addition to the
replacement and displacement of toxic bile acids, other mechanisms of action
include cytoprotection of the injured bile duct epithelial cells
(cholangiocytes) against toxic effects of bile acids, inhibition of apotosis of
hepatocytes, immunomodulatory effects, and stimulation of bile secretion by
hepatocytes and cholangiocytes.
Pharmacodynamics
Lithocholic acid, when
administered chronically to animals, causes cholestatic liver injury that may
lead to death from liver failure in certain species unable to form sulfate
conjugates. Ursodiol is 7-dehydroxylated more slowly than chenodiol. For
equimolar doses of ursodiol and chenodiol, steady state levels of lithocholic
acid in biliary bile acids are lower during ursodiol administration than with
chenodiol administration. Humans and chimpanzees can sulfate lithocholic acid.
Although liver injury has not been associated with ursodiol therapy, a reduced
capacity to sulfate may exist in some individuals.
Pharmacokinetics
Ursodiol (UDCA) is normally
present as a minor fraction of the total bile acids in humans (about 5%). Following
oral administration, the majority of ursodiol is absorbed by passive diffusion
and its absorption is incomplete. Once absorbed, ursodiol undergoes hepatic
extraction to the extent of about 50% in the absence of liver disease. As the
severity of liver disease increases, the extent of extraction decreases. In the
liver, ursodiol is conjugated with glycine or taurine, then secreted into bile.
These conjugates of ursodiol are absorbed in the small intestine by passive and
active mechanisms. The conjugates can also be deconjugated in the ileum by
intestinal enzymes, leading to the formation of free ursodiol that can be
reabsorbed and reconjugated in the liver. Nonabsorbed ursodiol passes into the
colon where it is mostly 7-dehydroxylated to lithocholic acid. Some ursodiol is
epimerized to chenodiol (CDCA) via a 7-oxo intermediate. Chenodiol also
undergoes 7dehydroxylation to form lithocholic acid. These metabolites are
poorly soluble and excreted in the feces. A small portion of lithocholic acid
is reabsorbed, conjugated in the liver with glycine, or taurine and sulfated at
the 3 position. The resulting sulfated lithocholic acid conjugates are excreted
in bile and then lost in feces. In healthy subjects, at least 70% of ursodiol
(unconjugated) is bound to plasma protein. No information is available on the
binding of conjugated ursodiol to plasma protein in healthy subjects or PBC
patients. Its volume of distribution has not been determined, but is expected
to be small since the drug is mostly distributed in the bile and small
intestine. Ursodiol is excreted primarily in the feces. With treatment, urinary
excretion increases, but remains less than 1% except in severe cholestatic
liver disease. During chronic administration of ursodiol, it becomes a major
biliary and plasma bile acid. At a chronic dose of 13 to 15 mg/kg/day, ursodiol
constitutes 30-50% of biliary and plasma bile acids.
Clinical Studies
Efficacy of Ursodeoxycholic
Acid Administered at 13 to 15 mg/kg/day in 3 or 4 Divided Doses to PBC Patients
A U.S., multicenter,
randomized, double-blind, placebo-controlled study was conducted to evaluate
the efficacy of ursodeoxycholic acid at a dose of 13 to 15 mg/kg/day,
administered in 3 or 4 divided doses in 180 patients with PBC (78% received
four times a day dosage). Upon completion of the double-blind portion, all
patients entered an open-label active treatment extension phase.
Treatment failure, the main
efficacy end point measured during this study, was defined as death, need for
liver transplantation, histologic progression by two stages or to cirrhosis,
development of varices, ascites or encephalopathy, marked worsening of fatigue
or pruritus, inability to tolerate the drug, doubling of serum bilirubin and
voluntary withdrawal. After two years of double-blind treatment, the incidence
of treatment failure was significantly (p < 0.01)
reduced in the URSO 250 mg group (20 of 86 (23%)) as compared to the placebo
group (40 of 86 (47%)). Time to treatment
failure, which excluded doubling of serum bilirubin and voluntary withdrawal,
was also significantly (p < 0.001)
delayed in the URSO 250 treated group (n=86, 803.8±24.9 d vs. 641.1±24.4 d for
the placebo group (n=86) on average) regardless of either histologic stage or baseline bilirubin levels ( > 1.8 or ≤ 1.8
mg/dl).
Using a definition of treatment
failure, which excluded doubling of serum bilirubin and voluntary withdrawal,
time to treatment failure was significantly delayed in the URSO 250 group. In
comparison with placebo, treatment with URSO 250 resulted in a significant
improvement in the following serum hepatic biochemistries when compared to
baseline: total bilirubin, SGOT, alkaline phosphatase and IgM.
Efficacy of Ursodiol
Administered at 14 mg/kg/day as a Once Daily Dose to PBC Patients
A second study conducted in
Canada randomized 222 PBC patients to ursodiol, 14 mg/kg/day or placebo,
administered as a once daily dose in a double-blind manner during a two-year
period. At two years, a statistically significant (p < 0.001) difference between the two treatments (n=106
for the URSO 250 group and n=106 for the placebo group), in favor of ursodiol,
was demonstrated in the following: reduction in the proportion of patients
exhibiting a more than 50% increase in serum
bilirubin; median percent decrease in bilirubin (-17.12% for the URSO 250 group
vs. +20.00% for the placebo group), transaminases (-40.54% for the URSO 250
group vs. +5.71% for the placebo group) and alkaline phosphatase (-47.61% for
the URSO 250 group vs. -5.69% for the placebo group); incidence of treatment
failure; and time to treatment failure. The definition of treatment failure
included: discontinuing the study for any reason; a total serum bilirubin level
greater than or equal to 1.5 mg/dl or increasing to a level equal to or greater
than two times the baseline level; and the development of ascites or
encephalopathy. Evaluation of patients at 4 years or longer was inadequate due
to the high drop out rate (n=10 withdrew from the URSO 250 group vs. n=15 from
the placebo group) and small number of patients. Therefore, death, need for
liver transplantation, histological progression by two stages or to cirrhosis,
development of varices, ascites or encephalopathy, marked worsening of fatigue or
pruritus, inability to tolerate the drug, doubling of serum bilirubin and
voluntary withdrawal were not assessed.
Efficacy of URSO 250
Administered in Twice a Day Versus Four Times a Day Divided Dosing Schedules to
PBC Patients
A randomized, two-period
crossover study in fifty PBC patients compared efficacy of URSO 250 (ursodiol)
in twice a day versus four times a day divided dosing schedules in 50 patients
for 6 months in each crossover period. Mean percent changes from baseline in
liver test results and Mayo risk score (n=46) and serum enrichment with UDCA
(n=34) were not statistically significant with any dosage at any time interval.
This study demonstrated that UDCA (13 to 15 mg/kg/day) given twice a day is
equally effective to UDCA given four times a day. In addition, URSO 250 was
given as a single versus three times a day dosing schedules in 10 patients. Due
to the small number of patients in this arm of the study, it was not possible
to conduct statistical comparisons between these regimens.