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Effects of Topical Anticholinesterases on Procaine Hydrolysis
E F F E C T S O F TOPICAL ANTICHOLINESTERASES ON PROCAINE HYDROLYSIS P H I L I P P. ELLIS, M.D.,
AND KATHERINE LITTLEJOHN,
Denver,
B.S.
Colorado
termine this relationship. The term plasma cholinesterase is used in our study and some of the references cited, although a serum preparation was used in the laboratory stud ies. In previous reports, the terms plasma cholinesterase, serum cholinesterase, and pseudocholinesterase have been used synon ymously.
The administration of succinylcholine chloride as a muscle relaxant during general anesthesia is potentially dangerous in pa tients with atypical or lowered plasma cholinesterase.1"4 Enzymatic hydrolysis of suc cinylcholine is reduced to such a degree that apnea and prolonged respirator}' paralysis may occur. Patients receiving topical ocular anticholinesterase agents such as echothio phate iodide have lowered plasma cholinesterase levels.5'6 The administration of succinyl choline to patients receiving such topical anticholinesterases may produce respiratory complications.7'8 Plasma cholinesterase also catalyzes the hydrolysis of local anesthetic agents includ ing procaine hydrochloricle.9-10 Toxic reac tions to procaine administration occurred in a patient who had hereditary atypical plasma cholinesterase.11 Patients with reduced plasma cholinesterase as a result of systemic diseases such as toxic goiter, severe anemia, hepatic dysfunction, or any debilitating dis ease may have adverse reactions to smaller than usual toxic doses of procaine.12'13 Like wise, Leopold has mentioned (personal com munication) the possibility of a similar oc currence in patients receiving topical anti cholinesterase for treatment of glaucoma. The in vitro addition of certain anticholinesterases—physostigmine, neostigmine, and R02-683—to human serum has reduced pro caine hydrolysis.14'15 However, to our knowl edge, there have been no reports regarding the alteration of procaine hydrolysis from serum of patients on topical anticholinester ase therapy. In this study, we attempted to de-
MATERIALS AND METHODS
Serum preparation—Blood samples were drawn by venipuncture from 36 healthy nonglaucomatous adults, ages 42 to 69 years, and 20 glaucoma patients, ages 8 to 86 years (only one subject was less than 42 years), who were being treated with echothiophate iodide. Samples were allowed to clot at room temperature, refrigerated at least one hour, and centrifuged for five minutes at 400 g. The serum was decanted and refrigerated. We usually analyzed the samples for pro caine hydrolysis and pseudocholinesterase levels on the same day they were obtained; analyses were never delayed more than two days after the samples were obtained. Only samples free of hemolysate were used. Plasma cholinesterase analyses—Plasma cholinesterase levels were determined by he method of Ellman and associates16 using the Boehringer Mannheim Corporation cholin esterase test kit, No. 15984. The assay for serum cholinesterase depended on the hy drolysis of acetylthiocholine. Analysis was carried out at 32°C in a Beckman DU spectrophotometer at a wave length of 405 m[/, and a 1-cm light path. Procaine hydrolysis—Procaine hydrolysis by plasma cholinesterase was determined by the method of Kalow.10 This method was based on the ultraviolet absorption spectra of procaine and its chief hydrolytic product, para-aminobenzoic acid. The hydrolysis was
From the Division of Ophthalmology, University of Colorado Medical Center, Denver, Colorado. This work was supported in part by an unrestricted grant from Research to Prevent Blindness, Inc. Reprint requests to Philip P. Ellis, M.D., 4200 E. Ninth Ave., Denver, CO 80220. 71
72
AMERICAN JOURNAL OF OPHTHALMOLOGY
JANUARY, 1974
TABLE HYDROLYSIS OF ACETYLTHIOCHOLINE AND PROCAINE
Control group Echothiophate-treated glaucoma group % Reduction in echothiophate-treated group
Acetylthiocholine, /imol/min/ml Serum
Procaine, /imol/min/ml Serum
3.2611 ± . 8 2 9 1 * (36) 1.5553+.7711 (20) 52.3%
0 . 0 1 5 1 + .0036 (36) 0 . 0 0 7 4 + . 0 0 4 1 (20) 50.9%
* Values+ are standard deviations. Numbers in parentheses indicate number of samples used.
proportional to the decrease in optical den procaine per minute per milliliter of undiluted serum. The echothiophate-treated group sity during esterase action on procaine. In this study we used the following re showed a 52.3% decrease in hydrolysis of agents: 0.15M phosphate buffer pH 7.4, pro acetylthiocholine and a 50.9% decrease in hy caine HC1 10 |xg/ml in pH 7.4 buffer, and drolysis of procaine as compared, to the con serum 1:10 dilution with buffer. All re trol group (Table). The difference between agents were brought to a temperature of the two groups in the rate of hydrolysis of 32°C. Dilute serum, 1.5 ml, was mixed with each substrate is statistically significant, 1.5 ml of procaine solution and read against P <.005. 1.5 ml of buffer mixed with 1.5 ml of dilute Hydrolysis of procaine did not occur in serum. Spectrophotometer readings were phosphate buffer solution without serum. made at a wave length of 300 mil at 32°C with The addition of isoflurophate eliminated hy a 1-cm light path. We observed the reaction drolysis of procaine by serum. These results for five minutes and recorded the change in indicated that under our experimental condi optical density. The effective extinction coeffi tions, nonenzymatic self-hydrolysis of pro cient of procaine at 300 mu. was 1.394 X 104. caine did not occur. This coefficient was the difference in absorp No differences were found between male tion between procaine and para-aminobenzoic and female subjects in the rate of hydrolysis acid at this wave length. of either substrate. The mean age for our To eliminate the possibility of nonenzy- nonglaucomatous patients was 48.7 years matic self-hydrolysis of procaine, it was and the mean age for the nonglaucomatous mixed with phosphate buffer without serum, female patients was 51.1 years. No signifi and spectrophotometric analysis for procaine cant difference in plasma cholinesterase lev hydrolysis completed. Isoflurophate ( D F P ) , els was found between the two groups. an effective inhibitor of plasma cholinester- There was good correlation between the hy ase, was added in a concentration of 10~5M drolysis of procaine and plasma cholinester to serum; we then determined the effect on ase activity in both groups; the coefficient correlation (r) was 0.8316 in the nonglauco serum hydrolysis of procaine. matous group and 0.9047 in the echothio RESULTS phate-treated glaucoma group (Figs. 1 and 2). Serum from nonglaucomatous patients hyDISCUSSION drolyzed 3.2611 ± .8291 (S.D.) u,mol acetyl thiocholine and 0.0151 ± 0.0036 [/.mol pro Local anesthetics can be divided into two caine per minute per milliliter of serum. groups on the basis of their molecular struc Serum from echothiophate-treated glaucoma ture. In one group an ester linkage is present patients hydrolyzed 1.5553 ± 0.7711 [j.mol in the intermediate chain between the amino acetylthiocholine and 0.0074 ± 0.0041 (unol group and the aromatic ring. Examples of
VOL. 77, NO. 1
ANTICHOLINESTERASES AND PROCAINE HYDROLYSIS
this group are procaine (Novocain) hydrochloride, tetracaine (Pontocaine) hydrochloride, piperocaine (Metycaine) hydrochloride, chloroprocaine hydrochloride (Nesacaine), hexylcaine (Cyclaine) hydrochloride, and butacaine (Butyn) sulfate. Local anesthetics of the ester group are inac tivated by hydrolysis which is catalyzed by plasma cholinesterase.17 The second group of local anesthetics are those with amides located between the aro matic ring and the amino group in a variety of linkages. Examples of these drugs are lidocaine (Xylocaine) hydrochloride, dibucaine (Nupercaine) hydrochloride, and mepivacaine (Carbocaine) hydrochloride. These drugs are not significantly affected by plasma cholinesterase. The intravenous use of procaine as a car diac anti-arrhythmatic has largely been re placed by procainamide hydrochloride, an other amide-type agent not appreciably af fected by plasma cholinesterase. However, procaine is still widely used as a local anes thetic.
FM
ACETYLlHIOCHOUNE/MINUTE/ml SERUM
Fig. 1 (Ellis and Littlejohn). The relationship between acetylcholine hydrolysis (plasma cholines terase activity) and procaine hydrolysis in nonglaucomatous patients.
1.0
2.0
3.0
|jM ACETYlTHIOCHOUNE/MtNUTE/ml SERUM
Fig. 2 (Ellis and Littlejohn). The relationship between acetylcholine hydrolysis (plasma cholines terase activity) and procaine hydrolysis in echothiophate-treated glaucoma patients.
73
The maximum safe dose of procaine for most adults is 1,000 mg (approximately 14 mg/kg of body weight). Toxicity does not appear to increase with concentration; that is, the maximum safe dose is 100 ml of a 1% solution, 50 ml of a 2% solution, 25 ml of a 4% solution, and so forth. Concentrations above 2% are not necessary for infiltrative anesthesia; higher concentrations are some times used for regional blocks or for spinal anesthesia. In most clinical situations a dose much below that maximum safe dose is em ployed. Only in procedures such as extensive skin grafting, performed with infiltrative an esthesia, or when regional blocks are per formed with higher concentrations of pro caine, are doses approaching toxic levels used.18 In this study, the mean decrease in hydro lysis of procaine from serum of patients on echothiophate iodide was decreased approxi mately 5 1 % . This cannot be interpreted to indicate that in these patients the maximum safe dose of procaine is about 500 mg. Blood levels of drugs reflect not only the dose given and hydrolysis, but also vascularity of the site of administration, duration of drug administration, and redistribution of the drug throughout the entire body. Thus, al though the rate of hydrolysis plays a major role in reducing blood concentration below toxic levels, other factors are important. Therefore, absolute safe doses of a drug cannot be extrapolated from data on altera tion of enzyme hydrolysis alone. However, since procaine is primarily hydrolyzed by plasma cholinesterase, patients receiving echothiophate iodide therapy as well as other anticholinesterases affecting plasma cholin esterase certainly may exhibit toxic symp toms after normally safe doses of procaine. Although we measured only hydrolysis of procaine, hydrolysis of other local anesthet ics of the ester group such as tetracaine and piperocaine may correspondingly decrease in the plasma of patients receiving anticholinesterase therapy for glaucoma. Accord ingly; toxic manifestations might be encoun-
74
AMERICAN JOURNAL OF OPHTHALMOLOGY
tered with these anesthetic agents out of pro portion to the size of the dose in patients re ceiving topical anticholinesterases. Hydrolysis of procaine varies greatly with species since nonspecific esterases found in mammalian plasma are not identical.19 In rabbits, the level of procaine-hydrolyzing esterase appears to be related to the level of the enzyme atropinease.20 Plasma cholinesterase levels in infants less than 6 months old are less than in adults.21 Shanor and associates22 found that levels of plasma cholinesterase of young men (18 to 35 years) were higher than that in young women, but no difference existed in the 70- to 80-year-old group. We found no differences in any age group. The plasma of patients with atypical cholinesterases is less susceptible to inhibition by a number of inhibitors than normal cho linesterase is. Kalow and Genest23 used dif ferences in the effect of the inhibitor, dibucaine, to distinguish qualitative differences in atypical plasma pseudocholinesterase.23 24,25 Leopold has studied the inhibition of plasma cholinesterase of glaucoma patients with several inhibitors—dibucaine, echothiophate iodide, R02-683, and fluoride. Less inhibition of plasma cholinesterase oc curred in glaucoma patients not treated with echothiophate iodide than in the controls. Juul and Leopold26 described an unusual plasma cholinesterase faction by disk electrophoresis in a number of patients with chronic simple glaucoma.26 We made no at tempt to distinguish the plasma cholinester ase activity of glaucoma patients from plasma cholinesterase of nonglaucoma patients. SUMMARY
Procaine hydrolysis was decreased by ap proximately 5 1 % in a group of patients re ceiving topical echothiophate iodide for treatment of glaucoma. This decrease corre lated well with the decrease in plasma cholin esterase levels. In patients receiving topical ocular anti cholinesterases, toxic reactions to procaine hydrochloride or other ester-group local an
JANUARY, 1974
esthetics may occur with smaller than the normally safe doses. REFERENCES 1. Evans, F. T., Gray, P . W . S., Lehmann, H., and Silk, E . : Sensitivity to succinylcholine in rela tion to serum cholinesterase. Lancet 1:1229, 1952. 2. Kalow, W . : Cholinesterase types. In Wolstenholme, G. E. W., and O'Connor, C. M. (eds.) : Bio chemistry of Human Genetics (Ciba Foundation). London, Churchill, 1960, p. 39. 3. Foldes, F. F., Foldes, V. M , Smith, J. C , and Zsigmond, E. K.: The relation between plasma cho linesterase and prolonged apnea caused by succinyl choline. Anesthesiology 24:208, 1963. 4. Thompson, J. C , and Whittaker, M . : A study of the pseudocholinesterase in 78 cases of apnoea following suxamethonium. Acta Genet. 16:209, 1966. 5. Leopold, I. H . : Ocular cholinesterase and cho linesterase inhibitors. Am. J. Ophthalmol. 51:885, 1961. 6. de Roetth, A., Jr., Dettbarn, W. D., Rosen berg, P., Wilensky, J. G., and Wong, A . : Effect of Phospholine Iodide on blood cholinesterase levels of normal and glaucoma subjects. Am. J. Ophthalmol. 59:586,1965. 7. Pantuck, E. J . : Echothiophate iodide eye drops and prolonged response to suxamethonium. Br. J. Anaesth. 38 :406, 1966. 8. Gesztes, T . : Prolonged apnoea after suxame thonium injection associated with eye drops contain ing an anticholinesterase agent. Br. J. Anaesth. 38:408, 1966. 9. Brodie, B. B., Lief, P. A., and Poet, R.: The fate of procaine in man following its intravenous administration and methods for the estimation of procaine and diethylaminoethanol. J. Pharmacol. Exp. Ther. 94:359, 1948. 10. Kalow, W . : Hydrolysis of local anesthetics by human serum cholinesterase. J. Pharmacol. Exp. Ther. 104:122, 1952. 11. Downs, J. R.: Atypical cholinesterase activ ity. Its importance in dentistry. J. Oral Surg. 24: 256, 1966. 12. Wood-Smith, F. G., Stewart, H. C , and Vickers, M. D . : Drugs in Anesthetic Practice, 3rd ed. New York, Appleton-Century Crofts, 1968, p. 209. 13. Reidenberg, M. M., James, M., and Dring, G.: The rate of procaine hydrolysis in serum of normal subjects and diseased patients. Clin. Phar macol. Ther. 13:279, 1972. 14. Kisch, B . : The influence of prostigmine and related compounds on the procaine esterase activity. Exp. Med. Surg. 1:84, 1943. 15. Foldes, F. F., and Aven, M. H . : The inhibi tion of the hydrolysis of procaine and 2-chloroprocaine in plasma by neostigmine and dimethylcarbamate of (2-hydroxy-5-phenylbenzyl) trimethylammonium bromide (R02-683). J. Pharmacol. Exp. Ther. 105:253, 1952. 16. Ellman, G. L., Courtney, K. D., Andres, V.,
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ANTICHOLINESTERASES AND PROCAINE HYDROLYSIS
Jr., and Featherstone, R. M.: A new and rapid colorimetric determination of acetylcholinesterase activ ity. Biochem. Pharmacol. 7:88, 1961. 17. Greene, N. M.: The metabolism of drugs em ployed in anesthesia. Anesthesiology 29:327, 1968. 18. LaDu, B.: Plasma esterase activity and the metabolism of drugs with ester groups. Ann. N.Y. Acad. Sci. 179:684, 1971. 19. Aven, M. H., Light, A., and Foldes, F. F.: Hydrolysis of procaine in various mammalian plas mas. Fed. Proc. 12:299, 1953. 20. Daniell, H. B., Wade, A. E., and Millikan, F. F.: Effect of procainesterase levels on duration of procaine local anesthesia. J. Pharm. Sci. 53.1341, 1964. 21. Zsigmond, E. K., and Downs, J. R.: Plasma cholinesterase activity in newborns and infants.
75
Can. Anaesth. Soc. J. 18:278, 1971. 22. Shanor, S. P., Van Hees, G. R., Baart, N., Erdos, E. G., and Foldes, F. F.: The influence of age and sex on human plasma and red cell cholines terase. Am. J. Med. Sci. 242:357, 1961. 23. Kalow, W., and Genest, K.: A method for the detection of atypical forms of human serum cholinesterase. Determination of dibucaine numbers. Can. J. Biochem. 35 :339, 1957. 24. Leopold, I. H., and Haddad, H. M.: Studies on serum pseudocholinesterase in glaucoma. Am. J. Ophthalmol. 61.1222, 1966. 25. Leopold, I. H.: New dimensions in ocular pharmacology. Am. J. Ophthalmol. 62:396, 1966. 26. Juul, P., and Leopold, I. H.: Studies on plasma cholinesterase isoenzymes in glaucoma. Am. J. Ophthalmol. 65 :527, 1968.
AND KATHERINE LITTLEJOHN,
Denver,
B.S.
Colorado
termine this relationship. The term plasma cholinesterase is used in our study and some of the references cited, although a serum preparation was used in the laboratory stud ies. In previous reports, the terms plasma cholinesterase, serum cholinesterase, and pseudocholinesterase have been used synon ymously.
The administration of succinylcholine chloride as a muscle relaxant during general anesthesia is potentially dangerous in pa tients with atypical or lowered plasma cholinesterase.1"4 Enzymatic hydrolysis of suc cinylcholine is reduced to such a degree that apnea and prolonged respirator}' paralysis may occur. Patients receiving topical ocular anticholinesterase agents such as echothio phate iodide have lowered plasma cholinesterase levels.5'6 The administration of succinyl choline to patients receiving such topical anticholinesterases may produce respiratory complications.7'8 Plasma cholinesterase also catalyzes the hydrolysis of local anesthetic agents includ ing procaine hydrochloricle.9-10 Toxic reac tions to procaine administration occurred in a patient who had hereditary atypical plasma cholinesterase.11 Patients with reduced plasma cholinesterase as a result of systemic diseases such as toxic goiter, severe anemia, hepatic dysfunction, or any debilitating dis ease may have adverse reactions to smaller than usual toxic doses of procaine.12'13 Like wise, Leopold has mentioned (personal com munication) the possibility of a similar oc currence in patients receiving topical anti cholinesterase for treatment of glaucoma. The in vitro addition of certain anticholinesterases—physostigmine, neostigmine, and R02-683—to human serum has reduced pro caine hydrolysis.14'15 However, to our knowl edge, there have been no reports regarding the alteration of procaine hydrolysis from serum of patients on topical anticholinester ase therapy. In this study, we attempted to de-
MATERIALS AND METHODS
Serum preparation—Blood samples were drawn by venipuncture from 36 healthy nonglaucomatous adults, ages 42 to 69 years, and 20 glaucoma patients, ages 8 to 86 years (only one subject was less than 42 years), who were being treated with echothiophate iodide. Samples were allowed to clot at room temperature, refrigerated at least one hour, and centrifuged for five minutes at 400 g. The serum was decanted and refrigerated. We usually analyzed the samples for pro caine hydrolysis and pseudocholinesterase levels on the same day they were obtained; analyses were never delayed more than two days after the samples were obtained. Only samples free of hemolysate were used. Plasma cholinesterase analyses—Plasma cholinesterase levels were determined by he method of Ellman and associates16 using the Boehringer Mannheim Corporation cholin esterase test kit, No. 15984. The assay for serum cholinesterase depended on the hy drolysis of acetylthiocholine. Analysis was carried out at 32°C in a Beckman DU spectrophotometer at a wave length of 405 m[/, and a 1-cm light path. Procaine hydrolysis—Procaine hydrolysis by plasma cholinesterase was determined by the method of Kalow.10 This method was based on the ultraviolet absorption spectra of procaine and its chief hydrolytic product, para-aminobenzoic acid. The hydrolysis was
From the Division of Ophthalmology, University of Colorado Medical Center, Denver, Colorado. This work was supported in part by an unrestricted grant from Research to Prevent Blindness, Inc. Reprint requests to Philip P. Ellis, M.D., 4200 E. Ninth Ave., Denver, CO 80220. 71
72
AMERICAN JOURNAL OF OPHTHALMOLOGY
JANUARY, 1974
TABLE HYDROLYSIS OF ACETYLTHIOCHOLINE AND PROCAINE
Control group Echothiophate-treated glaucoma group % Reduction in echothiophate-treated group
Acetylthiocholine, /imol/min/ml Serum
Procaine, /imol/min/ml Serum
3.2611 ± . 8 2 9 1 * (36) 1.5553+.7711 (20) 52.3%
0 . 0 1 5 1 + .0036 (36) 0 . 0 0 7 4 + . 0 0 4 1 (20) 50.9%
* Values+ are standard deviations. Numbers in parentheses indicate number of samples used.
proportional to the decrease in optical den procaine per minute per milliliter of undiluted serum. The echothiophate-treated group sity during esterase action on procaine. In this study we used the following re showed a 52.3% decrease in hydrolysis of agents: 0.15M phosphate buffer pH 7.4, pro acetylthiocholine and a 50.9% decrease in hy caine HC1 10 |xg/ml in pH 7.4 buffer, and drolysis of procaine as compared, to the con serum 1:10 dilution with buffer. All re trol group (Table). The difference between agents were brought to a temperature of the two groups in the rate of hydrolysis of 32°C. Dilute serum, 1.5 ml, was mixed with each substrate is statistically significant, 1.5 ml of procaine solution and read against P <.005. 1.5 ml of buffer mixed with 1.5 ml of dilute Hydrolysis of procaine did not occur in serum. Spectrophotometer readings were phosphate buffer solution without serum. made at a wave length of 300 mil at 32°C with The addition of isoflurophate eliminated hy a 1-cm light path. We observed the reaction drolysis of procaine by serum. These results for five minutes and recorded the change in indicated that under our experimental condi optical density. The effective extinction coeffi tions, nonenzymatic self-hydrolysis of pro cient of procaine at 300 mu. was 1.394 X 104. caine did not occur. This coefficient was the difference in absorp No differences were found between male tion between procaine and para-aminobenzoic and female subjects in the rate of hydrolysis acid at this wave length. of either substrate. The mean age for our To eliminate the possibility of nonenzy- nonglaucomatous patients was 48.7 years matic self-hydrolysis of procaine, it was and the mean age for the nonglaucomatous mixed with phosphate buffer without serum, female patients was 51.1 years. No signifi and spectrophotometric analysis for procaine cant difference in plasma cholinesterase lev hydrolysis completed. Isoflurophate ( D F P ) , els was found between the two groups. an effective inhibitor of plasma cholinester- There was good correlation between the hy ase, was added in a concentration of 10~5M drolysis of procaine and plasma cholinester to serum; we then determined the effect on ase activity in both groups; the coefficient correlation (r) was 0.8316 in the nonglauco serum hydrolysis of procaine. matous group and 0.9047 in the echothio RESULTS phate-treated glaucoma group (Figs. 1 and 2). Serum from nonglaucomatous patients hyDISCUSSION drolyzed 3.2611 ± .8291 (S.D.) u,mol acetyl thiocholine and 0.0151 ± 0.0036 [/.mol pro Local anesthetics can be divided into two caine per minute per milliliter of serum. groups on the basis of their molecular struc Serum from echothiophate-treated glaucoma ture. In one group an ester linkage is present patients hydrolyzed 1.5553 ± 0.7711 [j.mol in the intermediate chain between the amino acetylthiocholine and 0.0074 ± 0.0041 (unol group and the aromatic ring. Examples of
VOL. 77, NO. 1
ANTICHOLINESTERASES AND PROCAINE HYDROLYSIS
this group are procaine (Novocain) hydrochloride, tetracaine (Pontocaine) hydrochloride, piperocaine (Metycaine) hydrochloride, chloroprocaine hydrochloride (Nesacaine), hexylcaine (Cyclaine) hydrochloride, and butacaine (Butyn) sulfate. Local anesthetics of the ester group are inac tivated by hydrolysis which is catalyzed by plasma cholinesterase.17 The second group of local anesthetics are those with amides located between the aro matic ring and the amino group in a variety of linkages. Examples of these drugs are lidocaine (Xylocaine) hydrochloride, dibucaine (Nupercaine) hydrochloride, and mepivacaine (Carbocaine) hydrochloride. These drugs are not significantly affected by plasma cholinesterase. The intravenous use of procaine as a car diac anti-arrhythmatic has largely been re placed by procainamide hydrochloride, an other amide-type agent not appreciably af fected by plasma cholinesterase. However, procaine is still widely used as a local anes thetic.
FM
ACETYLlHIOCHOUNE/MINUTE/ml SERUM
Fig. 1 (Ellis and Littlejohn). The relationship between acetylcholine hydrolysis (plasma cholines terase activity) and procaine hydrolysis in nonglaucomatous patients.
1.0
2.0
3.0
|jM ACETYlTHIOCHOUNE/MtNUTE/ml SERUM
Fig. 2 (Ellis and Littlejohn). The relationship between acetylcholine hydrolysis (plasma cholines terase activity) and procaine hydrolysis in echothiophate-treated glaucoma patients.
73
The maximum safe dose of procaine for most adults is 1,000 mg (approximately 14 mg/kg of body weight). Toxicity does not appear to increase with concentration; that is, the maximum safe dose is 100 ml of a 1% solution, 50 ml of a 2% solution, 25 ml of a 4% solution, and so forth. Concentrations above 2% are not necessary for infiltrative anesthesia; higher concentrations are some times used for regional blocks or for spinal anesthesia. In most clinical situations a dose much below that maximum safe dose is em ployed. Only in procedures such as extensive skin grafting, performed with infiltrative an esthesia, or when regional blocks are per formed with higher concentrations of pro caine, are doses approaching toxic levels used.18 In this study, the mean decrease in hydro lysis of procaine from serum of patients on echothiophate iodide was decreased approxi mately 5 1 % . This cannot be interpreted to indicate that in these patients the maximum safe dose of procaine is about 500 mg. Blood levels of drugs reflect not only the dose given and hydrolysis, but also vascularity of the site of administration, duration of drug administration, and redistribution of the drug throughout the entire body. Thus, al though the rate of hydrolysis plays a major role in reducing blood concentration below toxic levels, other factors are important. Therefore, absolute safe doses of a drug cannot be extrapolated from data on altera tion of enzyme hydrolysis alone. However, since procaine is primarily hydrolyzed by plasma cholinesterase, patients receiving echothiophate iodide therapy as well as other anticholinesterases affecting plasma cholin esterase certainly may exhibit toxic symp toms after normally safe doses of procaine. Although we measured only hydrolysis of procaine, hydrolysis of other local anesthet ics of the ester group such as tetracaine and piperocaine may correspondingly decrease in the plasma of patients receiving anticholinesterase therapy for glaucoma. Accord ingly; toxic manifestations might be encoun-
74
AMERICAN JOURNAL OF OPHTHALMOLOGY
tered with these anesthetic agents out of pro portion to the size of the dose in patients re ceiving topical anticholinesterases. Hydrolysis of procaine varies greatly with species since nonspecific esterases found in mammalian plasma are not identical.19 In rabbits, the level of procaine-hydrolyzing esterase appears to be related to the level of the enzyme atropinease.20 Plasma cholinesterase levels in infants less than 6 months old are less than in adults.21 Shanor and associates22 found that levels of plasma cholinesterase of young men (18 to 35 years) were higher than that in young women, but no difference existed in the 70- to 80-year-old group. We found no differences in any age group. The plasma of patients with atypical cholinesterases is less susceptible to inhibition by a number of inhibitors than normal cho linesterase is. Kalow and Genest23 used dif ferences in the effect of the inhibitor, dibucaine, to distinguish qualitative differences in atypical plasma pseudocholinesterase.23 24,25 Leopold has studied the inhibition of plasma cholinesterase of glaucoma patients with several inhibitors—dibucaine, echothiophate iodide, R02-683, and fluoride. Less inhibition of plasma cholinesterase oc curred in glaucoma patients not treated with echothiophate iodide than in the controls. Juul and Leopold26 described an unusual plasma cholinesterase faction by disk electrophoresis in a number of patients with chronic simple glaucoma.26 We made no at tempt to distinguish the plasma cholinester ase activity of glaucoma patients from plasma cholinesterase of nonglaucoma patients. SUMMARY
Procaine hydrolysis was decreased by ap proximately 5 1 % in a group of patients re ceiving topical echothiophate iodide for treatment of glaucoma. This decrease corre lated well with the decrease in plasma cholin esterase levels. In patients receiving topical ocular anti cholinesterases, toxic reactions to procaine hydrochloride or other ester-group local an
JANUARY, 1974
esthetics may occur with smaller than the normally safe doses. REFERENCES 1. Evans, F. T., Gray, P . W . S., Lehmann, H., and Silk, E . : Sensitivity to succinylcholine in rela tion to serum cholinesterase. Lancet 1:1229, 1952. 2. Kalow, W . : Cholinesterase types. In Wolstenholme, G. E. W., and O'Connor, C. M. (eds.) : Bio chemistry of Human Genetics (Ciba Foundation). London, Churchill, 1960, p. 39. 3. Foldes, F. F., Foldes, V. M , Smith, J. C , and Zsigmond, E. K.: The relation between plasma cho linesterase and prolonged apnea caused by succinyl choline. Anesthesiology 24:208, 1963. 4. Thompson, J. C , and Whittaker, M . : A study of the pseudocholinesterase in 78 cases of apnoea following suxamethonium. Acta Genet. 16:209, 1966. 5. Leopold, I. H . : Ocular cholinesterase and cho linesterase inhibitors. Am. J. Ophthalmol. 51:885, 1961. 6. de Roetth, A., Jr., Dettbarn, W. D., Rosen berg, P., Wilensky, J. G., and Wong, A . : Effect of Phospholine Iodide on blood cholinesterase levels of normal and glaucoma subjects. Am. J. Ophthalmol. 59:586,1965. 7. Pantuck, E. J . : Echothiophate iodide eye drops and prolonged response to suxamethonium. Br. J. Anaesth. 38 :406, 1966. 8. Gesztes, T . : Prolonged apnoea after suxame thonium injection associated with eye drops contain ing an anticholinesterase agent. Br. J. Anaesth. 38:408, 1966. 9. Brodie, B. B., Lief, P. A., and Poet, R.: The fate of procaine in man following its intravenous administration and methods for the estimation of procaine and diethylaminoethanol. J. Pharmacol. Exp. Ther. 94:359, 1948. 10. Kalow, W . : Hydrolysis of local anesthetics by human serum cholinesterase. J. Pharmacol. Exp. Ther. 104:122, 1952. 11. Downs, J. R.: Atypical cholinesterase activ ity. Its importance in dentistry. J. Oral Surg. 24: 256, 1966. 12. Wood-Smith, F. G., Stewart, H. C , and Vickers, M. D . : Drugs in Anesthetic Practice, 3rd ed. New York, Appleton-Century Crofts, 1968, p. 209. 13. Reidenberg, M. M., James, M., and Dring, G.: The rate of procaine hydrolysis in serum of normal subjects and diseased patients. Clin. Phar macol. Ther. 13:279, 1972. 14. Kisch, B . : The influence of prostigmine and related compounds on the procaine esterase activity. Exp. Med. Surg. 1:84, 1943. 15. Foldes, F. F., and Aven, M. H . : The inhibi tion of the hydrolysis of procaine and 2-chloroprocaine in plasma by neostigmine and dimethylcarbamate of (2-hydroxy-5-phenylbenzyl) trimethylammonium bromide (R02-683). J. Pharmacol. Exp. Ther. 105:253, 1952. 16. Ellman, G. L., Courtney, K. D., Andres, V.,
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