Absorption of Heparin-Heparinoids

HEPARIN and synthetic heparinoids have to be administered parenterally to assure clinical efficacy. Since oral administration of these drugs would be highly desirable, numerous compounds were tested for their ability to effect absorption from the gastrointestinal tract. The criterion of absorption was the appearance of plasma lipemia-clearing activity, which was first described by Hahn.

Several compounds (‘adjuvants’) were discovered to have this property, the best of these being certain salts of ethylenediaminetetraacetic acid (EDTA). [Emphasis added.] Other active compounds will be the subjects of future communications. The present communication describes the use of salts of EDTA to obtain absorption of heparin and synthetic heparinoids.

Rats weighing about 200gm were placed in individual cages and fed for 72hr. a liquid diet consisting of 30ml. of a 33 percent casein hydrolysate and water ad lib. The drug was given by intubation of aqueous solution at a volume of 10ml/kgm.

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At specified times, the rats were anesthetized with pentobarbital sodium, and blood samples taken by cardiac puncture.

In dogs, after an overnight fast, the drugs were given orally in gelatin capsules. Blood samples were taken by venipuncture at specified intervals. Lipemia-clearing activity of citrated plasma was determined and expressed in clearing units (C.U.) calculated according to Grossman, except that incubation time was limited to 10 minutes and the substrate was ‘Ediol’ (SchenLabs), diluted 1 to 50 with water. Whole-blood clotting time was determined by a three-tube Lee-White method and indicated in minutes. When blood did not clot within 60 min, a value of 60 min was used in calculating mean clotting time, but the ‘>’ sign was added. All values for clearing activity and clotting time are given as mean ą standard deviation for groups of 5-10 animals or as specified.

The drugs used were heparin sodium U.S.P., potassium salt of sulphopolyglucin (SPG) and various salts of EDTA. SPG is a sulphated polysaccharide [Emphasis added.] of 5 to 15 glucose units connected by a, 1-4, and a, 1-6 linkages, with 2-3 sulphate groups per glucose unit.

Typical experiments demonstrating the adjuvant property of sodium EDTA are presented in Table 1. Using lipemia-clearing activity as an index of absorption, the result show, without a question, the adjuvant properties of sodium EDTA. In dogs, where SPG alone produces measurable although somewhat erratic clearing activity, absorption of the heparinoid is increased about ten-fold by simultaneous administration of the sodium salt of EDTA. [Emphasis added.] In rats, the effect of EDTA is even greater, since SPG alone is practically not absorbed.

Similar results with SPG have been found in every animal species studied (monkey, dog, cat, rabbit, and rat), as well as in man. The adjuvant property of EDTA has also been demonstrated for other synthetic heparinoids [Emphasis added.] (polyethylene sulphonate, dextran sulphate and others).

The adjuvant property [Emphasis added.] of the sodium salt of EDTA [Emphasis added.] can also be demonstrated for heparin, using both clotting time and clearing activity [Emphasis added.] as criteria of absorption (Table 2).

The effects of EDTA are again unequivocal. In rats, clearing activity reached a peak about 30 min. after administration of the drug combination, and then declined slowly to control-levels. A similar time course was obtained for the anti-coagulant effect, with doubling of the clotting [Emphasis added.] time at about 30 min. after the drug and subsequent return to control-levels. Since it is known that heparin is not absorbed from the gastro-intestinal tract, control experiments without EDTA were not made.

In dogs, the typical heparin effects appeared slower than in the rates. The peak of clearing activity and anti-coagulant effects were reached about 2 hr. after drug combination and returned to control-levels after more than 4 hr.

Further experiments were carried out in dogs to establish dose-response curves for SPG, both with and without EDTA. The results in Fig. 1 show good correlation between dose and clearing as well as anti-coagulant activity. Statistical evaluation indicated that the efficacy of SPG was increased approximately five-fold when EDTA, in the ratio 1:4, was used as adjuvant. [Emphasis added.]

Significant amounts of heparin and synthetic heparinoids can be absorbed from the gastro-intestinal tract when these drugs are given orally together with an alkali salt of EDTA. The mechanism of this adjuvant effect is not clear. [Emphasis added.] There is evidence that the Chelation of calcium and/or magnesium ions of EDTA may be involved for the following reasons: (1) Sodium, potassium and ammonium salts of EDTA are effective as adjuvants; #2) calcium and magnesium EDTA are not effective; (3) adding a calcium salt blocks the adjuvant action of alkali EDTA; (4) phosphate buffers are not effective; (5) the adjuvant has to be given orally; intravenous injection of the adjuvant does not influence the absorption of SPG from the gastro-intestinal tract.