CLINICAL PHARMACOLOGY
Mechanism of Action
Trental (pentoxifylline) is a xanthine derivative. It
belongs to a group of vasoactive drugs which improve peripheral blood flow and
thus enhance peripheral tissue oxygenation. The mechanism by which Trental
achieves this effect has not been determined, but it is likely that the
following factors are involved:
- Trental, as with other xanthine derivatives, relaxes certain
smooth muscles including those of the peripheral vessels, thus causing
vasodilatation or preventing spasm. This action, however, may have a limited
role in patients with chronic obstructive arterial disease when peripheral
vessels are already maximally dilated.
- Trental improves flexibility of red blood cells. This
increase in the flexibility of red blood cells probably contributes to the
improvement of the ability of blood to flow through peripheral vessels
(haemorheologic action). This property was seen during in vitro and in vivo experiments
with Trental but the correlation between it and the clinical improvement of
patients with peripheral vascular diseases has not been determined.
- Trental promotes platelet deaggregation.
Improvement of red blood cell flexibility and platelet
deaggregation contribute to the decrease in blood viscosity.
Pharmacokinetics
Pentoxifylline is almost completely absorbed after oral
administration. The Trental 400 mg sustained release tablet showed an initial
peak plasma pentoxifylline concentration 2 to 3 hours post-administration. The
drug is extensively metabolized. The active main metabolite 1-(5- hydroxyhexyl)-3,7-dimethyl-xanthine
(metabolite I) is measurable in twice the concentration in plasma of that of
its parent substance. Biotransformation products are almost exclusively eliminated
by the kidneys.
Food intake before the administration of Trental delayed the
absorption but did not decrease it.
In vitro and in vivo Animal Data
Pharmacodynamics
In dogs, 10mg/kg/i.v pentoxifylline produced a short but
significant drop in BP. 5-15 mg/kg/i.v. pentoxifylline produced a dose related
increase in heart rate and decrease in peripheral resistance for 30-60 minutes.
In dogs, cats, and rats, after 1-3 mg/kg pentoxifylline i.v. the blood
pressure, heart rate and respiration remained practically unchanged whereas
higher doses of pentoxifylline (14-25 mg/kg/i.v.) caused a transient decrease
in blood pressure and an increase in heart rate. In rabbits pentoxifylline
(2-10 mg/kg/i.v.) produced a dose related fall in BP. In rabbits, cats and dogs
the respiration was slightly stimulated. The blood pressure response in cats
and rabbits after epinephrine was slightly inhibited by pentoxifylline. The
i.v. administration of pentoxifylline or aminophylline in doses of 3-10 mg/kg
to cats resulted in a 20 and 35 % increase on dp/dt respectively.
Femoral musculature circulation in cat, measured indirectly
by heat-conduction probe, was increased by pentoxifylline (10-50 mg/kg/p.o. and
1-20 mg/kg/i.v.) and papaverine (1 mg/kg/i.v) while aminophylline (1-10
mg/kg/i.v) was without effect. In hepatic circulation in cat, pentoxifylline (2
mg/kg/i.v) was as effective as papaverine (1 mg/kg/i.v.) in increasing blood flow.
In carotid artery blood of anaesthetized cat, pentoxifylline
(5 mg/kg/i.v.) produced a 5.8 mmHg increase in PO2 whereas papaverine, (1
mg/kg/i.v) produced a 4.0 mmHg increase, aminophylline 3 mg/kg/i.v. produced a
1 mmHg increase in PO2 and 5 mg/kg/i.v. reduce O2 tension 1mmHg.
Reserpine pre-treatment did not influence the positive
chronotropic effect of pentoxifylline in rats.
On isolated rabbit hind limb, pentoxifylline-induced
vasodilatation was comparable to acetylcholine-induced vasodilatation at equal
doses.
In isolated guinea pig heart preparation, pentoxifylline
(30-600 μg) produced no significant effect on contractility or heart rate
and small increase in coronary flow while aminophylline (30-808 μg)
produced a biphasic effect on coronary flow, slight negative inotropism and no
rate alteration. The activity of pentoxifylline on coronary flow was not
influenced by propanolol (7.5 μg). In isolated guinea pig tracheal chain,
the bronchodilator activity of pentoxifylline, was significantly greater than
aminophylline. The presence of propanolol 10-6 g/mL did not affect results.
Contractions induced in isolated guinea pig seminal vesicle
by epinephrine were reduced by pentoxifylline and by aminophylline in the same
concentration range.
Bronchospasm induced by i.v. acetylcholine in guinea pigs
was inhibited by 97%, and that induced by i.v. histamine inhibited by 100%, at
pentoxifylline doses of 50 mg/kg/i.v. and 20 mg/kg/i.v. respectively.
On rabbit aorta strip preparation both pentoxifylline and
aminophylline inhibited the NE-induced contraction.
The histamine-induced increase of capillary permeability in
rats was not influenced by 10 or 25 mg/kg pentoxifylline i.p.
Pentoxifylline given orally (25-100 mg/kg) to rats had no
influence on blood sugar while in rabbits given i.v. (10-50 mg/kg) the higher
dose pentoxifylline increased blood sugar from 100 to 187 mg% at 1 hour
post-dosing.
In comparison to aminophylline, the central stimulatory
effect of pentoxifylline in rats was significantly milder. Pentoxifylline (40
and 200 mg/kg/p.o.) did not prevent convulsions induced by nicotine in mice.
Pentoxifylline does not influence significantly the motility of mice and rats, food
consumption of rats, sleeping time after hexobarbital in rats and mice, ptosis,
sedation and hypothermia of mice caused by reserpine, catalepsy in rats induced
by perphenazine of fighting behaviour in mice. It has no anticonvulsive,
anti-inflammatory and local anaesthetic activity and exhibits only a slight
analgesic, cholorectic, diuretic and antitussive effect.
The results of in vitro studies in which pentoxifylline was
added to blood from human volunteers, and in vivo studies in which
pentoxifylline was given orally or intravenously to patients with peripheral
vascular disease indicate that pentoxifylline can improve impaired erythrocyte
flexibility. The possible mechanism involved in this effect are most likely
related to intracellular adenosine triphosphate (ATP) inasmuch as ATP depleted
cells have reduced flexibility and vice versa. Pentoxifylline raises
erythrocytes intracellular ATP concentrations. In another in vitro study using
rat erythrocytes, pentoxifylline has been shown to decrease intracellular Ca++ concentrations
and increase phosphorylation of the proteins in the erythrocytes membrane by
facilitating Mg++ dependent phosphoprotein phosphatase and transglutaminase activity.
This results in an increased membrane phosphoprotein concentration, which is
believed to increase red blood cell flexibility.
In an in vivo rat study designed to test platelet
deaggregation properties of drugs, pentoxifylline at doses of 3,6 and 12
mg/kg/i.v. reduced platelet aggregation to “sticky” cancer cells (Walker 256
carcinosarcoma) and inhibited their attachment to endothelium. Monkeys given pentoxifylline
6, 12, 18 and 24 mg/kg/i.v. exhibited dose related reduction in platelet
aggregation index. In human pentoxifylline inhibits ADP-stimulated platelet
aggregation as measured by the Born method.
Epinephrine-induced lipolysis (rat epididymal adipose
tissue) was increased by pentoxifylline and aminophylline at 10-3 and
10-4 M in vitro. In vivo, epinephrine-induced glycerine production (same
tissue) was significantly inhibited by both compounds (10 mg/kg/i.v.) and FFA
was decreased.
Pharmacokinetics
Beagle dogs were given 3.0 mg/kg/p.o pentoxifylline-14C and
radioactivity measured in plasma and body tissues. Mean maximal blood levels
(2.1 μg/mL) were reached 1 hour post-dosing. Plasma concentration/time
curve displayed a biphasic elimination profile with t1/2 0.8 hours and 30
hours. Over 80% of the radioactivity was found in urine within 24 hours. At
maximal blood levels time, highest concentration was found in gallbladder
(110.0 μg/g), kidney, liver and bladder (4.8 μg/g): lowest
concentrations were found in brain (0.40 μg/g), fat, heart and gonads (1.3
μg/g).
Toxicology
Acute Toxicity
ACUTE TOXICITY (LD50) OF PENTOXIFYLLINE
SPECIES |
ROUTE |
LD50(MG/KG) |
Mouse |
p.o |
1385 |
i.v. |
197 |
i.p |
239 |
Rat (SD) |
p.o |
1772 |
i.v. |
231 |
Toxicity was characterized by hypersalivation in orally
dosed animals, increased or irregular respiration, tonic-clonic convulsions and
paresis.
Rabbits survived 50 mg i.v; signs and symptoms of toxicity
were similar to those seen in rats. Dogs survived 160 mg i.v and 320 mg p.o.
They showed aggression and ataxia after oral dosing and aggression, fear, vomiting,
diarrhea after i.v dosing.
Subacute and Chronic Toxicity
Mouse i.v., 14 days:
Groups of 8 female 12 week old mice were given daily doses
of 0, 12.5, 25, 50 or 100 mg/kg of pentoxifylline. One mouse of the highest
dosage group died after 6 days. Death was preceded by dyspnea and clonic
convulsions. The other animals of this group showed a decrease in spontaneous
activity and had their eyes closed.
Mouse, p.o., 78 weeks:
Four groups of 20 males and females were given
pentoxifylline in diet at 0, 50, 150 or 450 mg/kg/day. Five animals per sex per
group were killed after 26 weeks and another 5 at 52 weeks. After 78 weeks the
remaining animals were observed for 13 weeks, without exposure to the compound.
High dose males showed a greater frequency of bronchiectasis, renal cysts,
testicular atrophy, urinary bladder dilatation and bone marrow hyperplasia than
controls. High dose females showed a greater frequency of bronchiectasis, fatty
degeneration of the liver, fatty degeneration/amyloidosis in the kidneys,
splenic hyperplasia, hyperplasia and fibrosis of bone marrow and osteoporosis
than controls.
There was an increased incidence of benign ovarian and
uterine tumours, and angiosarcoma of the liver was observed in 1 animal of each
sex in the high dose group.
Rat, i.v., 14 days:
Groups of 10 females were given pentoxifylline at daily
doses of 0, 12.5, 25, 50 or 100 mg/kg. Four of the 10 rats given 100 mg/kg
showed depressed spontaneous activity, staggering gait, closed eyelids,
salivation and clonic and tonic convulsions and died. There were pulmonary hemorrhages
in these 4 rats.
Rat, i.v, 30 days:
Groups of 10 males and 10 females were given pentoxifylline
in doses of 0, 10, 25 or 50 mg/kg/day. There was a slight decrease in
cholesterol and esterified cholesterol in the 25 and 50 mg/kg male groups and a
slight increase in the mean blood glucose level in the 25 and 50 mg/kg female
groups. Perilobular hyaline droplet degeneration of the liver occurred in all
groups, but appeared to be more severe in the male rats of the two highest
dosage groups. Females on the top dose displayed increased incidence of renal
tubule calcification.
Rat, p.o., 78 weeks:
Groups of 70 males and 70 females were given pentoxifylline
in their diet 0, 50, 150 or 450 mg/kg/day. Five animals per sex per group were
killed at 52 weeks and another 5 at 78 weeks. After 78 weeks the remaining
animals were observed for 26 weeks without additional exposure to
pentoxifylline. In the middle-dose group the body weight gain was significantly
decreased; at the end of the 6 months follow-up period the body weight were
normal. In the high-dosage group the body weight gain was decreased. At the end
of the 6 months follow-up period the female weight had returned to normal but
the males had not. The mortality rate was significantly increased for the males
in the high-dose group. The cause of death was similar in treated and untreated
animals, but in the treated animals there was an increase in congestive streaks
of the liver, cadiosclerosis and scars in the heart, dilatation of the uterus,
and thyroid atrophy (females only). There were more interstitial cell tumours
of the testicles in the high dosage group but the difference was not
significant. There was a significant increase in fibroadenomas of the mammary
gland (females) in the high dose group.
Dog, i.v., 30 days:
Groups of 3 male and 3 female Beagles were given
pentoxifylline in doses of 0, 10, 25 and 63 mg/kg 5days/week for 6 weeks. There
was licking of the lips, vomiting, incoordination, uneasiness and dose-related
heart rate increase following the injection. Some tubular renal degeneration
occurred at 25 and 63 mg/kg. There was also congestion of liver at these doses
and congestion of spleen at the highest dose.
Dog, p.o., 1 year:
Groups of 3 male and 3 female Beagles were given
pentoxifylline in doses of 0, 32, 100, 320 or 400 mg/kg/day. There was
incoordination, salivation and altered temperament following drug administration.
Deaths occurred at doses of 320 and 400 mg/kg due to extensive or focal pulmonary
oedema and hemorrhages, and marked congestion in mucosa of the intestinal
tract. Acetone was detected in urine at 2 weeks to 26 weeks in some dogs of the
3 highest dose groups. At 52 weeks acetone was no longer detected. Giant cell
formation in the testicles was observed in 2 dogs, which died in the 320 mg/kg
group. Granuloma in the lymph nodes occurred in 1 dog in the control group, and
2 in the 320 mg/kg group.
Reproduction and Teratology
Mouse, i.v.:
Mice were given 0, 12.5, 25 or 50 mg/kg pentoxifylline from
day 15 of gestation through day 21 of lactation. Between days 21 and 23 all the
animals were killed. Some of the F1 offspring were reared and mated. The
females and F2 offspring were raised to weaning, and then killed. All other F1 offspring
were killed at 10 weeks. There was no significant effect on pregnancy and on the
fetal development.
Rat, p.o.:
Groups of 10 males and 20 females were given 0, 57, 170 or
570 mg/kg/day pentoxifylline for 10 weeks before mating and then continuously
through gestation and lactation. Fifty percent of the females were killed on
the 13th day of gestation and the remaining animals were allowed to raise their
young to weaning.
The number of resorptions, particularly early resorption,
was greater in the high dose group. The number of young reared to weaning was
lower for the high dose group.
Rat, p.o. and i.v.:
Groups of 20 females were given pentoxifylline 0, 57, 100 or
570 mg/kg orally or 0.8, 3.2 or 12.5 mg/kg i.v. from the 6th or 7th day to the
16th day of gestation. Two control groups were used in the i.v. study. One
group was given a volume of physiological NaCl similar to the treatment groups
and the other group was not treated at all. On the 20th day of pregnancy the
fetuses were removed by Caesarean section. There was a significant reduction in
the number of fetuses in the highest oral dosage group and the number of resorption
sites was increased. There were no fetal abnormalities. The highest i.v. dose
caused a slight reduction in number of fetuses and increase in resorption.
Rat, p.o.:
Groups of 20-24 pregnant animals were given pentoxifylline
0, 57, 170 or 570mg/kg by stomach tube from day 17 of gestation to day 21
postpartum. Between days 21 and 23 all animals were killed. There were no drug
effects.
Rabbit, i.v. and p.o.:
Groups of 10 pregnant females were given pentoxifylline at
0, 26.5, 80 or 265 mg/kg/day orally or 1, 3.2, 0r 10 mg/kg/i.v./day. There were
no drug effects.
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