Upsala J Med Sci 79: 103-105, 1974 Improved Polymeric Contrast Agents for Roentgenologic Examination of the Gastrointestinal Tract 1. Preliminary Report on the Chemistry of the Polymers LARS BJORK, U N O E R I K S O N . BJORN I N G E L M A N and BIRGlTTA ZAAR From t h e D e p a r t m e n t s of Diagnostic Radiology a n d Clinical C h e m i s t r y , University Hospital, U p p s a l a , a n d Pharmucia AB, U p p s a l a , S w e d e n ABSTRACT Improved water-soluble iodine-containing polymers in- tended as contrast substances for use in the roent- genologic examination of the gastrointestinal tract have been synthesized. These polymers have a high solubility in water even at relatively low pH values, and, therefore, do not precipitate in the stomach. I N T R O D U C T I O N In an earlier publication in Znvestigative Radio- logy we reported on polymeric water-soluble iodine-containing contrast agents intended for use in roentgenologic examination of the gastrointesti- nal tract ( I ) . In contrast to the iodine-containing contrast agents used a t present, these polymeric contrast substances retain their solubility in water even a t relatively low pH values and therefore d o not precipitate in the stomach. T h e molecular weight and molecular dimensions a r e considerably larger than for the monomeric contrast substances used a t present f o r this purpose. Aqueous contrast solutions of t h e polymers a r e less hypertonic than solutions of the currently available contrast substances at comparable iodine concentrations. T h e diffusion coefficients of the polymers a r e lower than the diffusion coefficients of the mono- meric contrast agents. The advantages of water- soluble polymeric contrast agents of this type in certain instances in the roentgenologic examination of the gastrointestinal tract were briefly presented in o u r earlier paper ( I ) . In this report, improved polymeric contrast substances f o r this purpose are described. T h e synthesis method has been modified. Compared with o u r earlier polymers the improved polymers have an even higher solubility in water at low pH values. T h e average molecular weights M , and the molecular weight distributions have been changed slightly. Preparation qf wuter-soluble polymeric contrast sub- stances f o r examination qf the gastrointestinal tract T h e structure of the polymers reported in our previous paper ( I ) is presented in the schematic Fig. l a . In this figure -A- denotes iodine- substituted benzene derivatives (preferably 2, 4, 6- triiodobenzoic acid derivatives) a n d -B- denotes intermediate hydroxyl-bearing aliphatic bridges. T h e group -A- had for example t h e structure given in Fig. 1 b o r in l c . F o r contrast agents for oral use, the bridge -B- between t h e iodine- containing aromatic groups contained several a ) - A - 8 - A - B - A - B - A - B - A - B - A - C O O H COOH OH OH OH I OH d ) - C H , A H C H 2 O C H 2 h H C H , O ( C H 2 ) 4 0 C H 2 C H C H 2 0 C H 2 C H C H 2 - I Fig. 1 . ( a ) Basic structure o f the polymeric contrast substances. -A- indicates iodine-substituted benzene derivatives, mainly 2,4,6-triiodobenzoic acid derivatives. -B- indicates hydroxyl-bearing aliphatic bridges. ( 6 ) Example of group -A- ( c ) Example of group -A- ( d ) Example of the hydroxyl-bearing aliphatic bridge -B-. In the substance 730E, described in this report, group -A- was represented by the example given in ( c ) . The bridge -B- i n this substance was of the type shown in ( d ) , but the hydroxyl groups were partly replaced by glycerol ether groups. Wpsala J Med Sci 79 104 L . Bjijrk et al. -1. 10,000 30.000 hydroxyl groups so that the contrast polymer would be readily soluble in water, even a t low p H values. Thus, the bridge -B- was preferably of t h e type shown in Fig. Id. In order to further increase t h e solubility at low p H values, additional hydroxyl groups have now been introduced into the molecules by treat- ing t h e polymers with glycidol in an alkaline aqueous solution. In this reaction the hydroxyl groups of the bridge -B- a r e partly replaced by glycerol ether groups. Substitution a t the ends of t h e molecules can also occur. T h u s , a s a result of the reaction with glycidol the number of hydroxyl groups in the polymeric contrast mole- cules is increased. As an example, t h e synthesis of substance 730 E is described below. T h e group -A- in sub- stance 7 3 0 E has t h e formula given in Fig. I L'. T h e bridge -B- i n this substance is principally of the type shown in Id, but the hydroxyl- groups have been partly replaced by glycerol ether groups. Synthesis of substance 730 E 245.6 grams of 5-acetylamino-2,4,6-triiodo-N- methyl-isophtalic acid monoamide were suspended in 140 ml of 4 N aqueous N a O H . At 3 0 T , 51.2 ml of glycidol were slowly added dropwise during 5 hours with continuous.stirring. T h e reac- tion mixture was allowed t o stand for an addi- tional I hour a t 30°C and then f o r 16 hours a t about 20"C, after which 10 ml of 5 N aqueous N a O H was added. Then 70 ml of 1,4-butane- dioldiglycide ether were added dropwise, with continuous stirring, during 5 hours, a t 30°C, T h e reaction mixture was allowed t o stand for another I hour a t 30°C and then for 18 hours a t about 20°C. Thereafter, 100 ml of water w a s added and the mixture was stirred during 2 hours a t 30°C. T h e n 20 ml of glycidol were added dropwise, with continuous stirring, during 2 hours, a t 30°C. T h e reaction mixture was allowed t o stand for 2 hours a t 30°C and then for 23 hours a t about 20°C. With stirring, 6 N aqueous HCI was added drop- wise t o adjust the p H t o 1.6. T h e solution ivas kept a t this p H for 2 hours. N o precipitate was obtained. T h e solution was neutralized with 4 N aqueous N a O H to p H 7.0. 400 ml of water w a s added and then 2500 ml of acetone in order t o precipitate the pofymer. After o n e d a y , the super- natant w a s separated from t h e syrup-like lower phase containing the sodium salt of the polymeric polyacid formed in the reaction. 200 ml of water was added to the lower phase and then I000 ml of acetone. After o n e d a y , t h e supernatant was separated from t h e lower phase whereafter 3 more reprecipitations with acetone were carried o u t in t h e same manner. T h e precipithte w a s dried a t 50°C in a vacuum. T h e substance ob- tained was designated 730E and was used for the animal experiments described in part I 1 of this report. T h e yield was 292 grams. T h e product contained 0.8% NaCI. T h e iodine content of t h e sodium salt of the polymeric contrast substance obtained w a s 35.7%. T h e weight average mo- lecular weight (au), determined by light scatter- ing, was about 5000. Fig. 2 shows the smoothed o u t integral molelcular weight distribution curve of substance 7 3 0 E determined by Dr Granath by gel chromatography o n a mixture of Sephadex Upsala J Med Sci 79 Improvedpolymeric contrust agents. I. 105 Fig. 3. Differential molecular weight distri- bution curve. Substance 730 E. 10,000 20,000 30,000 M W G50 Fine and Sephadex G7S Fine. The con- centration of the contrast substance in the eluted fractions was estimated from t h e absorbance at 242 nm. Fig. 3 shows t h e differential molecular weight distribution curve obtained by graphical derivation of the integral distribution curve shown in Fig. 2 . ( T h e method has earlier been used for t h e determination of molecular-weight distribution curves for other polymers and has been described by Granath & Kvist ( 2 ) and Arturson & Granath (3 .) DISCUSSION T h e new polymeric contrast substances, for example s,ubstance 730 E , have improved solubility properties in water at low p H values. A relatively low average molecular weight was chosen for these polymers in these introductory tests. T h u s , the average degree of polymerization is low but can b e increased. T h e molecular weight distribu- tion was such that the upper limit lay well below the limit of permeability of the renal glomeruli. Thus, should t h e contrast agent escape from the gastrointestinal tract, e . g . , into t h e abdominal cavity through a perforation in the intestinal wall, it would b e easily excreted in the urine. If a higher average degree of polymerization is chosen molecules with a size above t h e renal threshold , c a n b e removed by fractionation methods. As these improved polymeric contrast sub- stances have the desired chemical and physical- chemical properties, tests in animals have been started. In part 11 of this report, preliminary studies of substance 730 E in animals are described. R E F E R E N C E S 1. Bjork, L., Erikson, U . & Ingelman, B . : Poly- meric contrast media for roentgenologic examina- tion of gastrointestinal tract. Investigative Radio- logy5: 142, 1970. 2. Granath, K . & Kvist. B.: Molecular weight distribution analysis by gel chromatography on Sephadex. J Chromatogr 28: 69, 1967. 3 . Arturson, G . and Granath, K . : Dextrans as test molecules in studies of the functional ultrastructure of biological membranes. Clin Chim Acta 37: 309, 1972. Received December 2 2 , 1973 Address for reprints: U . Eriksson, M. 0. Department of Diagnostic Radiology University Hospital S-750 14 Uppsala Sweden Upsala J M e d Sci 79