Adenine nucleotide effect on intraocular pressure: Involvement of the parasympathetic nervous system.

Nucleotides are present in the aqueous humor possibly exerting physiological effects on intraocular pressure (IOP). To determine the effect of nucleotides such as ATP and its related derivatives on IOP, New Zealand white rabbits were used. IOP was measured in rabbits treated topically either with saline (control) or with a single dose (10 microg/microL) of adenine nucleotides (ATP, 2-meS-ATP, ATP-gamma-S, alpha,beta-meADP, alpha,beta-meATP and beta,gamma-meATP). Those nucleotides reducing IOP (alpha,beta-meATP and beta,gamma-meATP) were then tested in concentrations ranging from 1 to 100 microg/microL to obtain the IC(50) value. Several antagonists for the P2 and adenosine A1 receptors (all at 10 microg/microL) were assayed 30 min before the application of the hypotensive nucleotide beta,gamma-meATP. To see whether the nucleotide was acting directly on the structures involved in aqueous humor dynamics or on the autonomic nerves controlling IOP, animal denervation and sympathetic (yohimbine and ICI-118,551 at 10 microg/microL) and parasympathetic (atropine and hexametonium at 10 microg/microL) receptors' antagonists were used 30 min before the instillation of beta,gamma-meATP. alpha,beta-meATP and beta,gamma-meATP decreased IOP to 60% of control value (basal IOP=23.2+/-1.3 mmHg), with IC(50) of 1.59+/-0.21 microg/microLand 0.56+/-0.62 microg/microL, which corresponds to 3mM and 1mM respectively. Denervation completely abolished the effect of beta,gamma-meATP. Sympathetic antagonists did not modify the hypotensive effect of beta,gamma-meATP, but parasympathetic antagonists were able to abolish it. Among the series of adenine nucleotide tested, alpha,beta-meATP and beta,gamma-meATP presented hypotensive actions on IOP. beta,gamma-meATP seems to stimulate cholinergic terminals being its final effect the IOP reduction. Therefore, these two nucleotides are interesting pharmacological tools for those pathologies related with high intraocular pressure.

Ophthalmic formulations of the intraocular hypotensive melatonin agent 5-MCA-NAT.

Melatonin is a hormone responsible for the regulation of circadian and seasonal rhythms. This hormone is synthesised in many tissues in the body including the eye, where it regulates important processes. During the recent years, the role of melatonin in the control of IOP has been investigated and it has been demonstrated that melatonin receptors are present and involved in the dynamics of the aqueous humour. 5-Methoxycarbonylamino-N-acetyltryptamine (5-MCA-NAT) is a selective MT3 melatonin receptor agonist. Topical application of this product produces a clear reduction in intraocular pressure (IOP) in New Zealand white rabbits and in glaucomatous monkeys. In this work, the potent ocular hypotensive 5-MCA-NAT has been dissolved in excipients used in currently marketed drug formulations. Until now, this melatonin analogue had been dissolved in either DMSO or ethanol neither of which is suitable for ocular topical application in humans. Solubility assays in the different solvents were performed by the observation of the presence of drug crystals under optical microscopy. 5-MCA-NAT was completely dissolved in propylene glycol (PG) and polyethylene glycol 300 (PEG 300) within 24h. Ophthalmic formulations were prepared from different ratios of PG:PBS and the commercialized Systane product. Quantification of 5-MCA-NAT in the vehicles was assessed by HPLC. In vitro cytotoxicity of the formulations was evaluated by the MTT method and in vivo tolerance of 5-MCA-NAT in the solvents was analyzed by biomicroscopy and specular microscopy. Systane and proportions of PG:PBS up to 10% of PG did not show cytotoxicity in human corneal limbal epithelial cells (HCLE). In vivo experiments showed that the higher the ocular tolerance, the less amount of PG present. The ocular hypotensive effect of 5-MCA-NAT dissolved in the new formulations was checked measuring IOP for 8h after instillation of the substance. The best effect lowering IOP was obtained with 5-MCA-NAT dissolved in PG and diluted with PBS (PG 1.43%) in which 5-MCA-NAT produced a reduction of 28.11+/-2.0% and the effect lasted about 7h. In conclusion, new formulations accepted for ocular topical treatments different from DMSO or ethanol were capable of dissolving the melatonin analogue 5-MCA-NAT, preserving its ocular hypotensive ability. Therefore, the use of 5-MCA-NAT may be possible in the treatment of ocular hypertension and glaucoma.