Cholesterol gallstone dissolution in bile: dissolution kinetics of crystalline (anhydrate and monohydrate) cholesterol with chenodeoxycholate, ursodeoxycholate, and their glycine and taurine conjugates.

The dissolution kinetics of the cotnn~on crystalline forms of cholesterol (anhydrate and monohydrate) were studied in free and conjugated chenodeoxycholate and ursodeoxycholate solutions by static disk, rotating disk, and powder dissolution techniques. 'The dissolution kinetics of both forms of cholesterol were found to be non-diffusion controlled, i.e., the detachment rate of cholesterol molecules from the solid crystals was rate-limiting. Cholesterol dissolution rates in chenodeoxycholate solutions were significantly faster than in ursodeoxycholate solutions, and micellar solubilities and dissolution rates of anhydrous cholesterol were appreciably larger than those of cholesterol monohydrate. I n free and conjugated chenodeoxycholate solutions, the higher solubilities of anhydrous cholesterol converged to the lower solubilities of the monohydrate within a few days, but i n ursodeoxycholate solutions "anhydrous" cholesterol remained metastable f o l weeks. <:holesterol dissolution rates were accelerated in proportion to increases in the bile salt and neutral electrolyte (NaCI) concentration and correlated closely with the cholesterol solubilizing capacity of the bile salts (free bile salts 9 glycine conjugates > taurine conjugates). 1)issolution rates were a l s o increased by increases i n tcrnperature (17-47"C), but no change was observed at the polymorphic crystalline transition temperature of anhyclrous cholesterol (39°C). Linear extrapolations of the temperature dependence o f ' the dissolution rate constants for both f'ornls of cholesterol converged at higher temperatures ; i n d intersected at 86.7"C. This temperature has been shown b y other methods to he the transition temperature wherc cholesterol nlonohydrate is convertcd t o t he anh>drous f o r m . Dissolution rates were retarded markedly by pr t ia l protonation of the bile salt suggesting that sparingly soluble bile acids and cholesterol rnay cornpete for micellar hinding sites.ml'hese studies suggest that nlicellar dissolution rates of cholestcrol gallstones during therapy with ursodeoxycholic acid should be appreciably slowetthan during therapy with chenodeoxycholic acid. Since he clinical efficacy o f gallstone dissolution w i t h either agent appears to be similar, it is coliclutled that ~,hysical-chemical nwchanisms other than micellar solubilization may be operative in ursodeoxycholate-induced gallstone dissolution.-Igimi, H., and M. C. Carey. Cholesterol gallstone dissolution in bile: dissolution kinetics of crystalline (anhydrate and monohydrate) cholesterol with chenodeoxycholate, ursodeoxycholate, and their glycine and taurine con,jugates.J. Lipid Res. 198 1 . 22: 254-270. Supplementary key words static disk apparatus . rotating disk apparatus . powder dissolution . equilibrium solubility . clissolution rate . dissolution rate constant . diffusion controlled dissolution . non-diftusion controlled dissolution . Noyes-Nernst equation . Levich equation Cholesterol, which is universally distributed in animal tissues, is both a vital and lethal sterol. It plays a key s t ructural and funct ional role in cell membranes, serum l ipoproteins, and in the mixed micelles o f ' bile. l'wo strategic tubular organ systems bear the brunt of excess cholesterol deposition: cholesterol monohydrate consti tutes the major component of m o s t gallstones ( 1 , 2 ) and cholesterol monohydrate together with cholesteryl esters composes the lipid lesion of atherosclerosis (3). Less commonly, cholesteryl esters may form liquid crystalline accumulations in the liver, gallbladder wall, and reticulo-endothelial systems (4). Abbreviations: CDC, chenodeoxycholate: U I X , ursodeoxycholate: C:, cholate; I)<:, deoxycholate: T , G-, prefixes indicate taurine and glycine conjugates, respectively: ChA, anhydrous rholestet-ol ( polytnorphic form not specified); ChM, cholesterol monohydrate: GLC, gas-liquid chromatography; TLX:, thin-layer chromatography: H P I X , high performance liquid chromatography. ' Atidt-eas co~-respontlence and rept-int requests t o this author at the Peter Bent Brigham Hospital (Division of Brigham and Women's Hospital, I I K . ) , 7 5 Francis Street, Boston, MA 021 15. 254 Journal of Lipid Research Volume 2 2 , 1981 by gest, on O cber 9, 2017 w w w .j.org D ow nladed fom Whereas reversal of deposited cholesterol in the arterial intima is not yet a practical therapeutic measure, considerable success has been achieved in recent years in dissolving cholesterol gallstones in the gallbladder and bile ducts by medical means. T h e ingestion of t w o specific bile acids, chenodeoxycholic (3a,7a-dihydroxy-5/3-cholanoic, CDC) acid o r its 7phydroxy epimer, ursodeoxycholic (Sa,7P-dihydroxy5/3-cholanoic, UDC) acid, decreases cholesterol secretion into bile (5), desaturates bile with cholesterol (5), and induces slow dissolution of gallstones in about a quarter to a third of patients (6). Since a detailed understanding of the dissolution kinetics of cholesterol from biological precipitates is of considerable theoretical as well as therapeutic importance, the aims of the present study are: i ) to identify quantitatively the kinetic factors which control the dissolution rates of gallstones with both CDC and UDC, especially since these bile acids differ markedly in their cholesterol solubilizing capacities (7), ii) to compare the dissolution kinetics of cholesterol monohydrate (ChM) with anhydrous cholesterol (Ch.4) (8) (even though the latter is not found in gallstones, it may well be the chemical form of cholesterol in biological membranes and in the interior of other macromolecular lipid aggregates),' and iii) t o identify and evaluate possible physical-chemical factors which might accelerate or retard gallstone disso1utic)n i n vivo. We establish that dissolution rates and maximum micellar solubilities of ChA are significantly greater than ChM irrespective of the bile salt species. ChA and ChM dissolution is much faster in solutions of CDC and its conjugates than in solutions of UDC and its con-jugates, and in each case the dissolution kinetics are non-diffusion controlled. Thus the 'stone" solution interfacial reaction is rate-limiting for gallstone dissolution by bile salts and this reaction is slower for ChM than for ChA. These data suggest that with our present inability to shake the gallbladder mechanically in situ the prospects for accelerating gallstone dissolution by chemical means are not good. EXPERIMENTAL PROCEDURE