Histamine H3 receptor-mediated inhibition of sympathetically evoked mydriasis in rats

Pilocarpine in the Treatment of Presbyopia: Progress, Issues, and Future Prospects

Presbyopia is a common age-related visual impairment. With the aging of the population, the incidence of presbyopia is increasing globally, becoming a worldwide public health concern. Treatment options for presbyopia include optical lens correction, surgical intervention, and pharmacological therapy. Pharmacological treatments for presbyopia are non-invasive, reversible, and have emerged over the past decade. Following the US Food and Drug Administration's approval of 1.25% pilocarpine for presbyopia, the use of pilocarpine and its compound formulations has gained increased attention, with some drugs entering clinical phase II/III trials ( www.clinicaltrials.gov ). Therefore, this article primarily describes and analyzes the progress of research on the use of pilocarpine and its compound formulations for presbyopia, as well as the challenges that remain to be addressed. The optimal dosage form, the optimal concentration, the long-term safety, and patient compliance should be further explored, and there is a lack of multi-center evidence-based medicine research to support it. The aim of this article is to provide a reference for researchers to conduct further in-depth investigations in this area.

Pharmacological Treatment of Presbyopia by Novel Binocularly Instilled Eye Drops: A Pilot Study

PURPOSE

The feasibility, in terms of safety and potential efficacy, of a new drug combination for binocular use as a noninvasive pharmacological solution for treating presbyopia was examined.

METHODS

Fourteen emmetropic presbyopic subjects (28 eyes) were given one drop of the preparation under study in each eye. For each patient, the uncorrected distance visual acuity, uncorrected near visual acuity, near and far refraction, best corrected visual acuity, best corrected far-near visual acuity, photopic and scotopic pupil size, Schirmer's test, endothelial cell count, intraocular pressure, keratometry, pachymetry, and anterior chamber depth were all performed or assessed prior to the administration of the eye drops and then 0.5, 1, 2, 3, 4, and 5 h, 1 week, and 1 month post-administration prospectively in each eye and binocularly.

RESULTS

The results showed that near uncorrected visual acuity improved by about 2-3 lines from baseline in each eye and binocularly. There was no degradation in uncorrected far vision in each eye and binocularly in any patient. Refractive measurements performed in this study showed there was a maximum myopic shift of just 0.5 D that progressively reduced and disappeared at 4 h.

CONCLUSION

The new topical drug treatment analyzed herein significantly improved near vision without affecting far vision. This binocular pharmacologic treatment of presbyopia has the potential to ameliorate the reading vision of presbyopes and possesses the advantages of a nonmonovision therapy. A randomized, controlled, double-masked clinical trial with a twice-a-day treatment schedule is ongoing at our institution.

FUNDING

This study was supported in part by the Spanish Ministry of Health, Instituto Carlos III, Red Temática de Investigación Oftalmológica (OFTALRED), and Fundación Oftalmológica Vejarano (Popayán, Colombia).

Regulation of Norepinephrine Release from Isolated Bovine Irides by Histamine

In the present study, we investigated the effect of histamine on sympathetic neurotransmission from isolated, superfused bovine irides. We also studied the pharmacology of prejunctional histamine receptors that regulate the release of norepinephrine (NE) from this tissue. The effect of exogenous histamine and various histamine receptor agonists was examined on the release of [(3)H]-norepinephrine ([(3)H]NE) triggered by electrical field stimulation using the Superfusion Method. Histamine receptor agonists caused a concentration-dependent inhibition of field-stimulated [(3)H]NE overflow with the following rank order of potency: imetit > histamine > R-alpha-methylhistamine. In all cases, the inhibitory action of histamine receptor agonists was attenuated at high concentrations of these compounds. The histamine receptor antagonists, clobenpropit (H(3)-antagonist/H(4)-agonist) and thioperamide (H(3)-antagonist) blocked the inhibitory response elicited by R-alpha-methylhistamine and imetit, respectively. Inhibitory effects of R-alpha-methylhistamine and clonidine were not additive suggesting that prejunctional H(3)- and alpha(2)-adrenoceptors coexist at neurotransmitter release sites. We conclude that histamine produces an inhibitory action on sympathetic neurotransmission in the bovine iris, an effect mimicked by selective H(3)-receptor agonists and blocked by H(3)-antagonists.

Pharmacological characterization of the novel histamine H3-receptor antagonist N-(3,5-dichlorophenyl)-N'-[[4-(1H-imidazol-4-ylmethyl)phenyl]-methyl]-urea (SCH 79687)

We present the pharmacological and pharmacokinetic profiles of a novel histamine H3 receptor antagonist, N-(3,5-dichlorophenyl)-N'-[[4-(1H-imidazol-4-ylmethyl)phenyl]-methyl]-urea (SCH 79687). The H3-receptor binding Ki values for SCH 79687 were 1.9 and 13 nM in the rat and guinea pig (GP), respectively. The Ki values for SCH 79687 at histamine H1 and H2 receptors were greater than 1 microM. SCH 79687 showed a 41- and 82-fold binding selectivity for the H3 receptor over alpha 2A-adrenoceptors and imidazoline I2, and >500-fold H3 selectivity compared with over 60 additional receptors. The pA2 value for SCH 79687 in the GP ileum electrical field-stimulated (EFS) contraction was 9.6 +/- 0.3. Similar H3 antagonist activity was observed in the EFS cryopreserved and fresh tissue isolated human saphenous vein (HSV) assays (pKb = 9.4 +/- 0.3 and 10.1 +/- 0.4). SCH 79687 (30 nM) did not block clonidine-induced inhibition of EFS-induced contractions in HSV. SCH 79687 (ED50 = 0.3 mg/kg i.v.) attenuated (R)-alpha-methylhistamine inhibition of sympathetic hypertensive responses in the GP. At the time of activity evaluation, the GP plasma SCH 79687 concentration was 25 ng/ml at the dose of 0.3 mg/kg i.v. In feline nasal studies, combined administration of SCH 79687 (3 mg/kg i.v.) and the H1-antagonist loratadine (3 mg/kg i.v.), at individual doses that do not produce decongestion, inhibited the compound 48/80-induced congestion by 47%. The alpha-adrenergic agonist phenylpropanolamine (PPA; 1 mg/kg i.v.) also attenuated compound 48/80 nasal responses by 42%. Unlike the H3/H1 combination that did not affect blood pressure (BP), PPA (1 mg/kg i.v.) significantly increased BP compared with control animals by a maximum of 31 mm Hg. Orally, SCH 79687 (10 mg/kg) plus loratadine (10 mg/kg) also produced decongestion without effects on BP. In pharmacokinetic studies, oral dosing with SCH 79687 in the rat (10 mg/kg) and monkey (3 mg/kg) achieved plasma Cmax and area under the curve values greater than 1.5 and 12.1 microg. h/ml, respectively. SCH 79687 is an orally active H3 antagonist with a good pharmacokinetic profile that, in combination with an H1 antagonist, demonstrates decongestant efficacy comparable with oral sympathomimetic decongestants but without hypertensive liabilities.