Corneal sequestration and delivery to ciliary process of six topically applied sulfonamides.

The accession to cornea, aqueous humor and ultimately ciliary process was studied for a series of 6 sulfonamides which differed in their free acid lipophilicities. The drugs were applied as 5 mM solutions at pH 5 in unionized form or in chiefly ionized form at pH 8.4-9.4. These latter solutions contained 10% unionized drug. At 30 minutes post dose corneal drug levels were 2-3 fold higher for ionized compared to unionized application for a given drug. For ionized application there was a direct correlation between corneal drug accumulation and the lipophilicity of the unionized form of the drug. Cornea and aqueous humor drug levels were nearly equal 30 minutes following unionized application but significantly higher in cornea compared to aqueous humor after ionized application, suggesting drug was being retained in the cornea in ionized form. The efficiency of drug accession to ciliary process from cornea was assessed for the different drugs. The greatest accession to ciliary process from cornea as measured by the ratio of the two drug levels was found to correspond to compounds with free acid CHCl3/buffer partition coefficients approximately equal to unity. These results are discussed in terms of possible pathways of drug diffusion in the eye and the known intraocular pressure effects of topically applied sulfonamides.

Comparison of intraocular pressure lowering by topical and systemic carbonic anhydrase inhibitors in the rabbit.

We compared the effect on intraocular pressure (IOP) of maximal doses of a topical carbonic anhydrase inhibitor (CAI) at acidic and alkaline pH where it is maximally effective with full systemic CA inhibition in ocular normotensive New Zealand Albino rabbits. Tonometric IOP levels were measured hourly during 3 hour control period. Topical MK-417 (pKa 5.8, 8.3), a close congener of MK-507 (Dorzolamide) was given as a 1.4% solution at pH 4.5 (n=6) and pH 9.2 (n=6). MK-417 was instilled to the left eye with the right eye used as an untreated control. One hour later methazolamide was given intravenously at 10 mg/kg, a dose known to give full inhibition of the enzyme. Control IOP (mm Hg) was 19.12+/-0.50. One hour following MK-417, the left eye IOP was 13.40 +/-0.70 (pH 4.5) and 13.25+/-0.70 (pH 9.2). The right eye pressure was unchanged. Methazolamide injection at this time gave no further drop in the left eye IOP at either pH. IOP in the right eye fell to 14.00+/-0.70 so that 2 hours after methazolamide injection, the 2 eyes had the same pressure. In conclusion, topical CAI in sufficient dose and correct pH yields IOP lowering equivalent to a maximally effective dose of systemic CAI in rabbits.

Renal sulfate secretion is carbonic anhydrase dependent in a marine teleost, Pleuronectes americanus.

Though chemical assays indicate that carbonic anhydrase (CA) activity is present in marine teleost nephrons, CA inhibitors have no effect on urine pH or bicarbonate excretion, parameters typically CA dependent in almost all vertebrate groups. Because marine teleost renal sulfate secretion is associated with bicarbonate anion exchange, we investigated the effect of CA inhibition on transepithelial sulfate transport by flounder renal tubule primary monolayer cultures (PTC) and on renal sulfate secretion (QSO4) by intact flounder. Both methazolamide and ethoxzolamide (10 microM) inhibited PTC secretory flux by approximately 50%; reabsorptive sulfate flux, Na-dependent glucose transport, and transepithelial electrical resistance were unaffected. A CA inhibitor restricted to the extracellular space (10 microM polyoxyethylene-aminobenzolamide, 3.7 kDa) had no effect on PTC sulfate transport. Intravenous administration of methazolamide reduced QSO4 almost 40% and had no effect on glomerular filtration rate (GFR), urine flow rate, or Pi excretion rate. Serum pH was significantly reduced 0.2 units, whereas urine pH was unchanged. Together, the in vitro and in vivo results indicate that CA facilitates renal sulfate secretion in the seawater teleost.

The ocular distribution of methazolamide after corneal and scleral administration: the effect of ionization state.

The accession of methazolamide in ionized and unionized form to cornea, sclera, aqueous humor and ciliary process was studied 10 minutes following separate application to either sclera or cornea of a 1 mM solution. Cornea and ciliary process concentrations were 27.3 and 14.5 microM for unionized application to cornea and 10.1 and 7.1 microM for ionized application. Bulk aqueous humor concentrations were much lower, 3.1 and 1.1 microM, and cannot account for drug found in ciliary process on this time scale. Scleral application of drug, by contrast, gave undetectable ciliary process concentrations. These results are presented as a model for drug disposition following single drop topical sulfonamide therapy. The scleral pathway for drug delivery to ciliary process was further tested by application to sclera of 300 microL of either a 1 mM ionized or unionized solution for 30 minutes or as a 2% (85.5 mM) ionized solution for 30 minutes. In these series, red cell carbonic anhydrase was presaturated at -24 hours with drug to remove a possible route for loss of drug from sclera, that of the systemic circulation and the high concentration of carbonic anhydrase in red cells. After either ionized or unionized application, approximately 1 microM was detected in ciliary process, but all drug was attributable to the blood content of the tissue and the drug bound to red cell carbonic anhydrase. After 2% dosing, 4 microM was detected in ciliary process after allowance for drug in red cells. This concentration is below that necessary for inhibition of ciliary process carbonic anhydrase, suggesting that especially with regard to topical sulfonamide therapy, the corneal route of drug delivery to ciliary process predominates greatly over the scleral route.

Sulfonamides and secretion of aqueous humor.

The connection between carbonic anhydrase and the formation of aqueous humor arose in the decade 1950-1960 when a number of experiments of differing types showed that HCO3- ion catalytically formed in the ciliary process from CO2 was a principal element in the production of the aqueous. It was soon shown that inhibitors of the enzyme, given systemically, slowed HCO3- formation and sodium and fluid transport and thereby lowered intraocular flow and pressure in normal vertebrates and in patients with glaucoma. In the past 15 years successful efforts have been made to produce sulfonamide inhibitors that reach the ciliary process after local application to the cornea. The effects of locally acting sulfonamides are largely due to their amphoteric properties. They are soluble in both lipid and non-lipid media and penetrate cornea and sclera to reach ciliary process. Potency against carbonic anhydrase II is of the order of K(I) = 10(-9) M, so that topical application inhibits essentially all enzyme in the processes and is an effective nontoxic treatment for glaucoma.

Timolol decreases aqueous humor flow but not Na+ movement from plasma to aqueous.

PURPOSE: To determine whether the well-known effect of timolol in reducing ocular pressure and aqueous humor (AH) flow is a function of reduced Na+ movement from plasma to aqueous. Previously, the authors have shown this to be the case for carbonic anhydrase inhibitors. METHODS: The rate of appearance of 22Na in rabbit posterior aqueous was measured 1 to 3 minutes after the intravenous injection (time T) of the isotope. One hour before this, the animals received one of the following: two drops of 0.5% timolol, two drops of 3.5% pilocarpine, or 25 mg/kg intravenous methazolamide. At 1 minute (T + 1), a posterior chamber sample was taken; 2 minutes later (T + 3) a second sample was removed from the fellow eye. The rate constant of sodium accession is simply the difference between the two counts/2 minutes. Aqueous flow was measured by dilution of sulfacetamide marker as described previously. RESULTS: The rate constant (k(in)) for sodium entering the posterior chamber was 0.036 +/- 0.004 minute-1 (n = 17). Corresponding to previous findings, methazolamide (25 mg/kg intravenous) reduced this to 0.023 +/- 0.003 minute-1 (n = 14). Conversely, timolol (two drops of 0.5% solution) had no effect on kin, which measured 0.037 +/- 0.004 minute-1 (n = 12). Similarly, as expected, pilocarpine had no effect on k(in) (0.035 +/- 0.003 minute-1). Control flow was 3.9 microliters/minute +/- 0.4; after timolol, 2.5 microliters/minute +/-0.1; after methazolamide, 2.4 microliters/minute +/-0.2; after pilocarpine, 3.6 microliters/minute +/- 0.2. These are converted to rate constants by dividing by volume of posterior aqueous (60 microliters). The control rate constant for fluid entry was 0.065 minute-1, 1.8-fold higher than for sodium. CONCLUSIONS: A central dogma of the formation of AH (and cerebrospinal fluid) is that fluid moves isotonically from plasma to AH or cerebrospinal fluid and, therefore, that rate constant k(in) for fluid and for sodium are approximately the same. In the authors' hands, the fluid constant was modestly higher than for sodium. This holds for normal function and also for the reduced k(in) for fluid and sodium after carbonic anhydrase inhibition. The k(in) for neither flow nor sodium was affected by pilocarpine. Surprisingly, however timolol, which reduces flow, had no effect on Na+ entry.

Ocular absorption, blood levels, and excretion of dorzolamide, a topically active carbonic anhydrase inhibitor.

Dorzolamide is a powerful inhibitor of carbonic anhydrase (CA) II that penetrates the sclera and cornea to reach the ciliary process and lowers formation of HCO3 and aqueous humor. The usual dose applied to the eye in treatment of glaucoma is 1 drop (30 microL of 2% solution) every 8 hr to each eye, or a total daily dose of 4 mg. On this regime, the red cells accumulated drug over a period of 8 days, reaching a value of 20-25 microM, which corresponds to the concentration of CA II in human red cells. This drug concentration persisted throughout the 18 months of application. The plasma concentration was 0.034 microM, or 1/700 that of the red cells. This plasma concentration corresponds to that calculated from the dilution of administered drug into body water. The data are well fitted into the equilibrium expression for KI of dorzolamide against CA II at 37 degrees C, as 8 x 10(-9) M. The red cells also contain a small amount (5 microM) of the N-des-ethyl metabolite, probably reflecting its modest binding to CA I. In the initial 8-day drug period, virtually none appeared in the urine since CA II sites were being filled. At steady state, renal excretion was 1.3 mg/day and the renal clearance 90 ml/min. These excretion numbers include the small (20%) amount of the des-ethyl metabolite of dorzolamide. The relation of these data to lowering of intraocular pressure is clear. By the systemic route, an inhibitor such as acetazolamide is effective when free drug concentration in plasma is 2.5 microM. In the case of topical drugs, as shown here, the plasma concentration is some 100 x lower, but the concentration in ciliary process is 2-10 microM, comparable to that following systemic drugs (1). In conclusion, the concentration in plasma (reflecting free drug) of dorzolamide is about 1/200 of that needed for systemic effects as seen following acetazolamide or methazolamide. Thus, there is a clear pharmacological basis for the lack of any physiological effects of ocular dorzolamide, except on the eye itself.

Is cyanate a carbonic anhydrase substrate?

A study was undertaken to investigate whether diverse carbonic anhydrase (CA) isozymes (both native Zn as well as cobalt-substituted) are able to catalyze the hydrolysis of anions such as cyanide, cyanate, and thiocyanate. A controversy exists between the crystallographic and spectroscopic data of CA II-anion adducts. In the former case it has been shown that "metal poisons" such as CN- and CNO- are not directly coordinated to the active site Zn(II) ion whereas spectroscopic studies indicate otherwise. A theoretical study in the above systems did not resolve this controversy, since it was calculated that all three anions can act as CA substrates. In this paper we prove experimentally that none of them may act as substrates of CA and propose an explanation to the above controversy, discussing the mode of binding of small molecules within the enzyme active site.

Renal and cerebrospinal fluid formation pharmacology of a high molecular weight carbonic anhydrase inhibitor.

To achieve selective inhibition of cytosolic and membrane-bound carbonic anhydrase (CA II and CA IV, respectively), we synthesized a polymer of molecular weight 3500 from polyoxyethylene bis acetic acid and aminobenzolamide. The new compound, designated F (for Florida) 3500, is stable, water soluble and nontoxic. It is excreted largely unchanged by glomerular filtration. The Ki at 37 degrees C against CA II is 0.14 microM and against CA IV 4.0 microM, some 20 times more than the parent aminobenzolamide and about three times more than acetazolamide or methazolamide. F 3500 does not penetrate red cells and is confined to extracellular fluid. Relations of dose to renal HCO3-excretion was studied by i.v. injection into rats. Peak effect was reached with 100 mg/kg, eliciting 40 mM HCO3- in urine; this is taken to be the effect of inhibiting CA IV. This compares to a peak of 103 mM HCO3- after 3 mg/kg aminobenzolamide, which agrees with previous data on other low molecular weight sulfonamides and defines the effect on CA II and CA IV. We conclude that both isozymes are necessary for normal full renal reabsorption of HCO3-. We studied the effect of perfusing F 3500 (100-4000 microM) through the ventriculo-cisternal system of rats on cerebrospinal fluid (CSF) formation. F 3500 was also given intravenously at 100 mg/kg. CSF formation was unaffected. Low MW sulfonamides by either route lowered CSF formation 30 to 50%. It appears either that membrane-bound enzyme (CA IV) is not accessible from the CSF or blood side or that it plays little or no role in CSF formation.

Membrane carbonic anhydrase (IV) and ciliary epithelium. Carbonic anhydrase activity is present in the basolateral membranes of the non-pigmented ciliary epithelium of rabbit eyes.

Carbonic anhydrase inhibitors (CAIs) lower intraocular pressure by reducing aqueous flow. It has been thought that this pharmacologic reduction of aqueous flow is mediated by the ciliary epithelium, but it is not known whether this cellular action is effected by inhibition of the membranal (CA IV) and/or cytosolic (CA II) carbonic anhydrases of the ciliary epithelium. The isolated ciliary epithelial bilayer maintains its anatomic and functional polarity and generates a transepithelial potential difference (TEP) in an Ussing type chamber. Depletion of HCO3-, accomplished either with an HCO3(-)-free solution bathing the epithelial bilayer, or, with addition of freely permeant CAIs to HCO3(-)-containing media, (from either the PE or NPE side of the bilayer) depolarizes the preparation. Addition of CAIs to an HCO3(-)-depleted preparation has no further effect, indicating the specific action of the CAIs. The CAI, 2-p-NH2 benzenesulfonamido-1,3,4,-thiadiazole-5-SO2NH2, linked to polybutadiene maleic acid yields an impermeant polymer of 20000 Da with no loss of activity. At 45 microM this impermeant polymer caused a 60% increase in the SCC, seen only when the compound was applied to the NPE side of the bilayer. This latter result indicates an effect from inhibition of CA IV in the basolateral membranes of the NPE. Thus there are probably two different cellular actions of CAIs upon the ciliary epithelium to reduce aqueous inflow, cytoplasmic and membranal. The action of NPE basolateral membranal CA IV is probably linked to the chloride/bicarbonate exchanger.

[Topical carbonic anhydrase inhibitors in the treatment of glaucoma]. / Inhibiteurs locaux de l'anhydrase carbonique dans le traitement du glaucome.

Systemically administered carbonic anhydrase inhibitors have been in clinical use since the 1950's. Their use and side effects have been reviewed extensively. Our review summarizes the development of topically administered carbonic anhydrase inhibitors in patients with glaucoma. Moreover, it documents clinical trials leading to the present choice of dorzolamide (MK-507), which is now being reviewed for worldwide approval for marketing by 1995.

The effect of temperature on the binding of sulfonamides to carbonic anhydrase isoenzymes I, II, and IV.

We report the effect of temperature on the equilibrium dissociation constants (Kl) for a series of six sulfonamides binding to three carbonic anhydrase (CA) isoenzymes (I, II, and IV). Kl values obtained at 0 degree, 15 degrees, and 23 degrees under conditions of nearly constant and low substrate (CO2) concentration were used to calculate enthalpy and entropy changes associated with sulfonamide binding as well as to provide estimates of inhibitory potency of sulfonamides at 37 degrees. We studies four classic sulfonamide (methazolamide, benzolamide, ethoxzolamide, and sulfanilamide) and the novel sulfonamides MK-507 (dorzolamide) and CF3SO2NH2. In all cases, the Kl was observed to increase with increasing temperature, which is consistent with a negative enthalpy of sulfonamide binding. The extrapolated increase in Kl over the 0-37 degrees temperature range varied from 4-fold for sulfanilamide binding to CA l to 14-fold for CF3SO2NH2 binding to CA IV, corresponding to binding enthalpy values of -7.2 to -11.7 kcal/mol. For CA II and I, entropy changes associated with sulfonamide binding were in general modest and ranged from -5.3 to +4.1 entropy units (eu) for five of the compounds tested. In contrast, ethoxzolamide binding was associated with a relatively large positive entropy change. Also, the variatione in k(on) and k(off) with temperature were studied for three sulfonamides binding to CA II. The association rate constants for methazolamide, benzolamide, and ethoxzolamide binding showed increases of 2-fold or less, whereas dissociation constants increased 3-9-fold over the range of 0-37 degrees. Thus, the temperature effect in increasing Kl is in large part due to a faster rate of sulfonamide dissociation. Apparent activation parameters at 23 degrees for k(on) were delta H++ = -2.35 to 3.8 kcal/mol, delta G++ = 7.3 to 8.6 kcal/mol, and delta S++ = -16.2 to -32.7 entropy units. For k(off), the corresponding values were delta H++ = 5.6 to 14.5 kcal/mol, delta G++ = 19.0 kcal/mol, and delta S++ = -14.8 to -45.7 entropy units.

Effect of pH on the ocular distribution of a topical carbonic anhydrase inhibitor.

Ampholyte carbonic anhydrase inhibitors (including MK-927) have previously been shown to elicit greater intraocular pressure reduction when applied in ionized form at moderately acidic or alkaline pH compared to application of un-ionized drug at neutral pH. Since ionized solutions should be far more membrane-impermeant than un-ionized solutions, we attempted to solve this apparent paradox. We studied the distribution at 0.5-18 hr of MK-927 in eye tissues and fluid after topical instillation of one drop of a 0.5% solution at pH 4.9, 7.0 and 9.1. Measured drug concentrations at 30 min in corneal epithelium and stroma are approximately 4.5-fold greater after acidic or alkaline application than after neutral application, with the highest levels being found in corneal epithelium. The same pattern is seen in ciliary process, the site of aqueous humor production. Here drug concentration is approximately 60% of that in cornea at 30 min. Free drug concentrations in ciliary process were used to compute the time course of maximal carbonic anhydrase inhibition for the three modes of application. Drug concentration in sclera, uvea and aqueous humor at all times are all low by comparison, suggesting drug movement is from cornea to ciliary process via the corneo-scleral junction. The k(in) for proton movement across the corneal epithelium was measured (k(in) = 12.4 hr-1) from which a permeability coefficient (P = 2.7 x 10(-2) cm sec-1) was computed. Separate analysis was made of the pH status of the cornea 30 min and 1 hr following instillation of 1 drop 2% acidic (pH 4.9) and alkaline (pH 9.1) MK-927, with sodium sulfadiazine and pilocarpine.HCl as pH controls. Stromal bicarbonate at 30 min was approximately halved after acidic drops and doubled after alkaline drops consistent with pH decrease or increase of nearly 0.3 U. The results are in accord with classical schemes for amine permeation of the cornea at acidic pH when consideration is given to movement of acid equivalents and corneal pH. Thus drug inside the eye is in relatively (> 95%) lipophilic form except that trapped in the cornea shortly after acidic or alkaline applications.

Physiological and immunocytochemical evidence for a putative H-K-ATPase in elasmobranch renal acid secretion.

The mechanism of renal acid secretion in marine fish is largely unknown. We explored whether H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase) is present and functional in acid secretion in the kidney of the elasmobranch spiny dogfish shark, Squalus acanthias. In whole animal studies, a specific inhibitor of mammalian H(+)-K(+)-ATPase, Sch-28080, abolished greater than 87% of basal (62 mg/kg) and 75% of imidazole-stimulated titratable acid excretion (5 and 62 mg/kg). Antibodies directed against the COOH-terminus hog gastric H(+)-K(+)-ATPase alpha-subunit stained specific subdivisions of the neck, early and late proximal tubule, late intermediate tubule, both segments of the distal tubule, and the early collecting duct of the renal tubule of these fish. These findings are consistent with a major role for a protein similar to the mammalian gastric H(+)-K(+)-ATPase in elasmobranch renal acid secretion.

A new class of carbonic anhydrase inhibitor.

Aliphatic sulfonamides, as CH3SO2NH2, are very weak inhibitors of the carbonic anhydrases (KI congruent to 10(-4) M) and are extremely weak acids (Ka congruent to 10(-10.5) M). We now find CF3SO2NH2 a very potent inhibitor of carbonic anhydrase II (KI = 2 x 10(-9) M) and a much stronger acid (Ka = 10(-5.8) M). It freely dissolves in water, a 2% solution yielding pH 3.7, the strongest known sulfonamide acid. CHCl3/aqueous partition with this solution is very low, 0.006. The plot of CH3SO2NH2 and eight hydrophilic halo-alkyl congeners gives a linear relation over 5 orders of magnitude, pKI increasing as pKa declines. Transcorneal permeability of CF3SO2NH2 in rabbits is very high; from one drop on the cornea it rapidly gains access to the ciliary process, where it fully inhibits carbonic anhydrase and gives the maximum pressure drop (for any drug) of 6 mm Hg at 30-60 min. Action is short due to rapid disappearance of free drug from the eye at the rate of aqueous humor flow. Analyses are made of binding of CF3SO2NH2 to carbonic anhydrase and melanin in ciliary process. CF3SO2NH2 is not attacked by glutathione, is 75% bound to plasma protein, and is not taken into the renal secretory system. Excretion rate follows from glomerular filtration and tubular reabsorption. Intravenous injection thus leads to prolonged plasma levels (half-life, 24 h), a general diffusion into tissues and alkalinization of the urine, as with "classic" inhibitors. Drug is bound to carbonic anhydrase in red cells and decays with half-life of 4 days. The rapid and effective binding to carbonic anhydrase of this small hydrophilic molecule shows that complex lipophilic structures are not necessary for powerful inhibition of the enzyme. It does not appear essential for a sulfonamide to occupy a "hydrophobic pocket" in the active site cavity to react effectively at the zinc center.

Chemical properties of carbonic anhydrase IV, the membrane-bound enzyme.

The carbonic anhydrase (CA) isozyme (IV) in microsomes is thought to have a dominant role in secretory processes. Using microsomes from bovine kidney and lung (which had the same activity), we have measured the Km and kcat for CO2 hydration and compared these numbers with those for CA I (red blood cells and gut), CA II (red blood cells and secretory cells), and CA III (muscle). For kidney CA IV, Km is 10 mM and kcat is 170,000 sec-1 at 0 degree, approaching the rate for CA II but much greater than those for CA I or III. The Ki values for 11 sulfonamides with CA IV were measured and in all cases showed less binding (averaging 17-fold) than to CA II. This is the result of reduction of the association rate constants (k(on)), whereas the dissociation constants of the drug-enzyme complexes (k(off) are similar between CA II and IV. Based on these data, full physiological effects may be expected when inhibition of CA IV is about 99%. Anion inhibition of CA IV is similar to that of CA II and less than that of CA I or CA III. Data are compatible with the proposed role of CA IV in physiological events, i.e., HCO3- formation and secretion at one cell border and H+ separation and excretion at the other.

A comparison between the effect of topical and systemic carbonic anhydrase inhibitors on aqueous humor secretion.

We have assessed the onset and duration of decreased intraocular pressure and aqueous humor flow contrasting systemic and topical administration of carbonic anhydrase inhibitors. The relationship between physiological effects and fractional activity of carbonic anhydrase isoenzymes in the eye was also investigated. Experiments were performed in normotensive New Zealand white rabbits. Intraocular pressure was determined manometrically or tonometrically and aqueous humor flow by sulfacetamide clearance. We studied methazolamide (25 mg kg-1), ethoxzolamide (4 mg kg-1), and MK-927 (2% in 0.5% hydroxyethylcellulose, topical, pH 4.8). There is an immediate reduction in intraocular pressure (1.2 and 1.8 mmHg by 2 min) and aqueous flow (33% and 40% by 5 min) following intravenous dosing with either methazolamide or ethoxzolamide. This correlates with rapid appearance of drug in the anterior uvea and very low fractional activity of ocular carbonic anhydrase isoenzymes II (cytosolic) and IV (membrane bound). Peak intraocular pressure reduction averaged 4.2 +/- 0.68 mmHg and 4.5 +/- 0.8 mmHg for methazolamide and ethoxzolamide at 60 and 45 min, respectively. Peak flow reduction was 38% for methazolamide and 40% for ethoxzolamide, at 5 min. Aqueous flow and intraocular pressure returned to baseline at 7 and 4 hr following methazolamide and ethoxzolamide, respectively. This corresponds to decay of drug from ocular tissues and significant increases in fractional activity of carbonic anhydrase isoenzymes. Topical MK-927 resulted in a 1.2 mmHg decrease in pressure by 5 min. This correlated with the early appearance of drug in the anterior uvea prior to its appearance in aqueous humor and very low fractional activity of carbonic anhydrase isoenzymes. Intraocular pressure decreased 3.6 +/- 0.35 mmHg at 1 hr and returned to baseline by 6 hr. Aqueous flow was reduced 12% by 5 min and 35% at 1 hr. The appearance of MK-927 in the anterior uvea prior to detection in aqueous suggests a significant non-corneal route of absorption following topical administration. Topical MK-927 results in a more gradual reduction in intraocular pressure and flow, although peak effects are not statistically different from systemic carbonic anhydrase inhibitors. The time of pressure return to baseline is also comparable to systemic carbonic anhydrase inhibitors. Because the relations between carbonic anhydrase II and carbonic anhydrase IV in the ciliary process are not yet clear and since the drugs have different affinities for the isozymes, the precise degree of fractional inhibition necessary for pharmacological effect is not certain, but based on drug concentration in the anterior uvea, may take 98% inhibition for full intraocular pressure reduction.

pH and drug ionization affects ocular pressure lowering of topical carbonic anhydrase inhibitors.

PURPOSE: To evaluate the effect of drug ionization on the ocular hypotensive activity of topical carbonic anhydrase inhibitors. METHODS: Ocular normotensive New Zealand albino and ocular hypertensive Dutch Belted pigmented rabbits were used. Tonometric intraocular pressure levels were taken after topical application of 50 microliters of drug (at various concentrations and pH values) to one eye with the contralateral eye used as an untreated control. The drugs tested were MK-927, L-662,583, and AHR-16329. Eye tissues were analyzed for drug by our enzymatic methods. RESULTS: In all cases, the more ionized the applied drug the greater the ocular hypotensive activity. Tissue distribution studies showed that there was more drug found in the eye after the ionized form of a drug was applied than that found after application of the less ionized forms. CONCLUSIONS: Increasing the ionization of three ampholyte topical carbonic anhydrase inhibitors increases their ocular hypotensive activity. These data taken with ocular disposition data suggest that ionized compounds of this type are more readily sequestered in the cornea, which serves as a drug depot for prolonged drug delivery and activity.