23
Abstract
Phentolamine: pharmacokinetic aspects and pharmacodynamics in human corpus
cavernosum. In vivo and in vitro study.
Paper presented to the Course of Post-Graduation in Pharmacology of the Department of Physiology and
Pharmacology Of the Federal University of Ceara as a prerequisite for the gain of the doctor’s degree in
Pharmacology, Fortaleza, 2003.
Lucio Flavio Gonzaga Silva
Orientador: Prof. Dr. Manoel Odorico de Moraes
Introduction: Erectile dysfunction (ED) is defined as the Inability to achieve or maintain an erection
adequate for sexual satisfaction. Phentolamine an α-adrenergic antagonist has been used to treat ED since
1994, mostly in combination with other vasoactive agents. More recently oral formulation of Phentolamine
mesylate were developed for the disease. The drug is thought to relax penile smooth muscle by a inhibition
over α-receptors. Since the paper of Traish (1998) has been speculated that phentolamine may also relax
penile smooth muscle by a non-adrenergic mechanism.
The aim of this study is to understand the pharmacokinetics aspects of Phentolamine (in vivo study) using the
data from a bioequivalence test, and to investigate its pharmacodynamics with the purpose to clear its non-
adrenergic mechanism in human corpus cavernosum (in vitro study).
Methods (In vivo study): Thirty six healthy male volunteers (mean age 21,5 years old) were enrolled in the
study that consisted in a single dose, two-way randomized crossover design comparing one phentolamine
formulation (regitina®) to one standard phentolamine formulation (Vasomax®).
In vitro study: A total of 64 isolated human corporeal tissue strips obtained from 16 male donor cadaver (16 –
40 years old) were investigate. The pharmacologic activity of phentolamine-mediated relaxation, of pre-
contracted erectile tissue strips of human corpus cavernosum were studied in organ bath chambers(non-
adrenergic mean).
Results: (in vivo study): Regitina® 40 mg formulation C
max
geometric mean ratio was 108.29% (90% CI =
98.58 – 118.96 of Vasomax 40 mg formulation. Regitine® 40 mg formulation [AUC
(0-720 min)
] geometric mean
ratio was 102.33 (90% CI = 97.21 – 19= 07.72) of Vasomax® 40 mg formulation. The average phentolamine
pharmacokinetics parameters were C
max
15,4 ng/mL, T
max
50 min and t
1/2
3 h.
(In vitro study): Phentolamine caused concentration dependent relaxation in human corpus cavernosum strips
pre-contracted with the α-adrenergic agonist phenylephrine as well as with the non-adrenergic serotonin (10
-4
M), prostaglandin F
2α
(10
-4
M) and KCl (60 mM) agents, with the best efficacy against phenylephrine (100%
of relaxation at 10
-3
M - IC
50
= 1,5 x10-5M).
Tetrodotoxin (TTX – 10
-6
M) (Na
+
channel blocker) and atropine (10
-5
M) (muscarinic receptor inhibitor) did
not cause alterations in the phentolamine relaxation of the penile smooth muscle (54,6 ± 4,6% x 48,9 x 6,4%)
(52,7 ± 6,5% x 58,6 ± 5,6%) (p > 0,05).
The relaxation of phentolamine of the human corpus cavernosum strips pre-contracted with KCl (40 mM) was
significantly attenuated by N
G
-nitro-L-arginine L-NAME (10
-4
M) (NO synthase inhibitor) (59,7 ± 5,8% x
27,8 ± 7,1%) (p < 0,05) and 1H-[1,2,4] Oxadiazole [4,3-a]quinoxalin-1-one ODQ (10
-4
M) (inibidor da
guanilato ciclase (62,7 ± 5,1% x 26,8 ± 3,9%) (p < 0,05).
The role of the K channel blockers were investigated. Glibenclamide (10
-4
M) an inhibitor of ATP-activated
K
+
-channels (K
ATP
- inhibitor) caused a almost completely inhibition (90%) of the human corpus cavernosum
strips phentolamine relaxation, pre-contracted with KCl (40 mM) (56,7 ± 6,3% x 11,3 ± 2,3%) (P < 0,05).
Investigation with Glibenclamide + L-NAME did the same effect
(54,6 ± 5,6% x 5,7 ± 1,4%) (p < 0,05).
Charybdotoxin and apamin (blockers of CA
++
-activated K+ channels – K
ca
) did not alter the phentolamine
relaxations (54,6 ± 4,6% v 59,3 ± 5,2%)
Conclusion: The average phentolamine pharmacokinetics parameters were similar to the reported by
scientific literature. The two drugs are bioequivalents for the rate and extent of absorption.
The results from the Pharmacologic studies suggest that Phentolamine relaxes human corpus cavernosum by a
nonadrenergic noncholinergic mechanism activating the ATP-activated K
+
-channel (K
ATP
).
Keywords: Phentolamine, Pharmacokinetic, Pharmacodynamics, K
+
channel, erectile dysfunction.