Substrate Report for: Dipivefrin

We characterized the interaction of the prodrug dipivefrin hydrochloride (DPE) with esterase activity in the rabbit cornea. The esterases which were identified included: (1) cholinesterase, (2) acetylcholinesterase, (3) a mixture containing carboxylesterase, acetylesterase and arylesterase, and (4) a non-specific esterase. DPE suppressed all of their activities as well as that of the mixture containing carboxylesterase, acetylesterase and arylesterase, and a nonspecific esterase. However, its effect on cholinesterase was larger than on any of the other activities, suggesting that DPE is a better substrate for cholinesterase than for any of the other esterases. These measurements along with those of substrate-dependent inhibition of 14C-DPE hydrolysis indicated that the DPE-esterase interaction was competitive based on changes in the apparent Km values which were extracted from Lineweaver-Burk plots of esterase activity. The substrate for cholinesterase competed with DPE most strongly among substrates. These results seem to suggest that DPE is hydrolyzed by various corneal esterases, mainly cholinesterase.

Dipivefrin is an antiglaucoma prodrug that is hydrolyzed to the active drug, epinephrine, by esterases in the cornea. Since cholinergic antiglaucoma agents are frequently used in combination with adrenergic agents, it was of interest to determine the effects of a commonly used irreversible cholinesterase inhibitor, echothiophate (Phospholine) iodide, on the dipivefrin esterases. In vitro studies showed that echothiophate is a competitive, reversible inhibitor of the soluble corneal dipivefrin esterases. In vivo studies substantiated the reversible nature of echothiopate inhibition, since no inhibition of dipivefrin hydrolysis could be detected 1 3/4 hours after echothiophate treatment and as early as 15 minutes after dipivefrin application.

A time-course study was performed on the intraocular pressure response of pigmented rabbit eyes. Dipivefrin administration produced in initial hypertensive phase lasting less than two hours followed by a prolonged hypotensive phase. Echothiophate iodide therapy produced a more pronounced and prolonged hypertensive response; there was no hypotensive phase. Administration of echothiophate plus dipivefrin resulted in a hypertensive phase similar to that from echothiophate alone; as previously reported, this combination was not followed by a hypotensive phase. The alpha-blocker phentolamine mesylate prevented the echothiophate-induced hypertension. When dipivefrin was administered with echothiophate plus phentolamine, there was an immediate hypotensive effect. It was concluded that the hypertensive effect of echothiophate in pigmented rabbit eyes may mask the hypotensive action of dipivefrin. This, rather than an echothiophate-induced inhibition of esterases, may explain why combination therapy with these drugs seemed ineffective.

We characterized the interaction of the prodrug dipivefrin hydrochloride (DPE) with esterase activity in the rabbit cornea. The esterases which were identified included: (1) cholinesterase, (2) acetylcholinesterase, (3) a mixture containing carboxylesterase, acetylesterase and arylesterase, and (4) a non-specific esterase. DPE suppressed all of their activities as well as that of the mixture containing carboxylesterase, acetylesterase and arylesterase, and a nonspecific esterase. However, its effect on cholinesterase was larger than on any of the other activities, suggesting that DPE is a better substrate for cholinesterase than for any of the other esterases. These measurements along with those of substrate-dependent inhibition of 14C-DPE hydrolysis indicated that the DPE-esterase interaction was competitive based on changes in the apparent Km values which were extracted from Lineweaver-Burk plots of esterase activity. The substrate for cholinesterase competed with DPE most strongly among substrates. These results seem to suggest that DPE is hydrolyzed by various corneal esterases, mainly cholinesterase.

Dipivefrin is an antiglaucoma prodrug that is hydrolyzed to the active drug, epinephrine, by esterases in the cornea. Since cholinergic antiglaucoma agents are frequently used in combination with adrenergic agents, it was of interest to determine the effects of a commonly used irreversible cholinesterase inhibitor, echothiophate (Phospholine) iodide, on the dipivefrin esterases. In vitro studies showed that echothiophate is a competitive, reversible inhibitor of the soluble corneal dipivefrin esterases. In vivo studies substantiated the reversible nature of echothiopate inhibition, since no inhibition of dipivefrin hydrolysis could be detected 1 3/4 hours after echothiophate treatment and as early as 15 minutes after dipivefrin application.

A time-course study was performed on the intraocular pressure response of pigmented rabbit eyes. Dipivefrin administration produced in initial hypertensive phase lasting less than two hours followed by a prolonged hypotensive phase. Echothiophate iodide therapy produced a more pronounced and prolonged hypertensive response; there was no hypotensive phase. Administration of echothiophate plus dipivefrin resulted in a hypertensive phase similar to that from echothiophate alone; as previously reported, this combination was not followed by a hypotensive phase. The alpha-blocker phentolamine mesylate prevented the echothiophate-induced hypertension. When dipivefrin was administered with echothiophate plus phentolamine, there was an immediate hypotensive effect. It was concluded that the hypertensive effect of echothiophate in pigmented rabbit eyes may mask the hypotensive action of dipivefrin. This, rather than an echothiophate-induced inhibition of esterases, may explain why combination therapy with these drugs seemed ineffective.