Factors Affecting Gastrointestinal Absorption of Drugs Ruth R. Levine, PhD

Absorption from the intestine is dependent on: (1) the physiochemical state of the substance, (2) the nonabsorptive physiologic functions and state of the intestine, (3) the metabolic activity and functions of the absorbing cell, and (4) the structure of the absorbing surface. Certain changes within each of the four categories will produce uniform or predictable changes in drug absorption which may be applied generally. Other alterations produce highly predictable effects on absorption of certain classes of drugs, while still other alterations, particularly those in metabolic activity, produce changes which are specific for a single agent or group of structurally related agents. Unintentional alterations in either the rate or amount of drug absorption can lead to ineffective oral therapy on the one hand, or toxic reactions on the other.

Absorption [roln the intestines, whether of drugs or nutrients, is d e p e n d e n t on: (1) the physicochemical state of the sub. stance, (2) the nonabsorptive physiologic functions and state of the gastrointestinal 'xact, (3) the metabolic activity and functions of the absorbing cell, and (4) the structure of the absorbing surface. Changes in absorption, whether intended or unintended, are, in general, the consequence of alterations in one or m o r e of these several determinants. Therefore, our understanding of drug absorption and our ability to alter the absorption of a particular agent are related to our knowledge of the From tile Department of Pharmacology and Experimental Therapeutics, Boston University Medical Center, Boston, Mass. Supported in part by USPHS Grant NB-01966, and the Air Force Office of Scientific Research Grant 914. Address for reprint requests: Dr. Ruth R. Levine, Department of Pharmacology and Experimental Therapeutics. Boston University Medical Center, Boston, Mass.

Digestive Diseases,Vol. 15, No. 2 (February1970)

way each of these d e t e r m i n a n t s influences the overall process. In the following, we discuss under each of the four categories those factors which produce predictable effects on absorption of drugs in general, those factors which produce highly predictable effects on the absorption of certain broad classes of drugs, and those factors which produce effects specifically on a single agent or a small g r o u p of structttrally related agents. Examples will be cited for their value in illustrating the point u n d e r discussion but no complete literature review is attempted. Within each category we also a t t e m p t to define those factors which m a y be important, even essential, to o u r understanding of d r u g absorption but a b o u t which we have little knowledge.

Factors Related to Physicochemical Properties of the Drug A great deal of i n f o r m a t i o n is available on the relationship between absorption and

171

LEVINE: GASTROINTESTINALABSORPTION OF DRUGS

physicochemical properties of drugs and there is general agreement that absorption by the process of passive diffusion is largely governed by factors of molecular size and shape, degree of ionization, and tipoid solubility of the drugs, 1-4 It follows then that changes in tile physiocochemical state of a given agent are more likely to affect its absorption if absorption is primarily a passive rather than a biochemical process. Changes in the physicochemical state of an agent will affect its absorption regardless of the transport mechanism involved only when a drug is administered as a solid and has a rate of dissolution slower than its rate of absorption. 5 Although alterations in drug properties have provided the most productive and exploitable means for altering drug absorption. only a few such changes affect the absorption of an agent once it is in solution. Discussion is confined to the latter condition since the numerous ways pharmaceutical chemists have changed absorption by changing tlm form or formulation of a drug have been a d e q u a t e l y reviewed elsewhere.6-n T h e p H partition hypothesis predicts that increasing the p i t at tile site from which absorption occurs would decrease the absorption of weak acids and increase that of weak bases since the proportion of the more readily diffusable unionized moiety would decrease in the case of acids and increase for bases, v-'-~* For example, such p H effects on absorption have been brought about in vivo by the use of acetazotamide. 1~ T h e carbonic anhydrase iuhibitot was found to decrease the p H of the rat ileum and as a result, the absorption of D-amphetamine was decreased, that of salicylic acid was increased, and that of urea remained unchanged.l~ W h e t h e r such p H effects on absorption of a drug will 172

have therapeutic consequences, however, is dependent on postabsorption factors since, in general, changes in p H alter only the rate of absorption but have little or no effect on the efficiency (or total amount) of absorption.~ Under the dynamic conditions obtaining in vivo, the absorbed material is usually removed so rapidly by the circulator)' systems that tile equilibrium is constantly shifted to favor continued supply of the more readily diffusable unionized drug. Under physiologic conditions, moreover, what appear to be deviations from the predicted results of the p H partition hypothesis are frequently only manifestations of the complex nature of the total process of absorption. For example, it would be anticipated that phenobarbital and pentobarbital, acids with pK a values of 7.2 TM and 8.1, .7 respectively, would he more rapidly absorbed from the stomach than from the small intestine. T h e reverse is true, however, w i t h 2-3 times the percentage absorbed from the stomach in 1 hr being absorbed from tile small intestine in about 10 rain (Table 1, Control column).as T h e much greater surface area of the intestinal mucosa relative to that of the gastric mucosa va,20 more than compensates for the decreased rate of a b s o r p t i o n / u n i t area o[ intestine. T h a t - t h i s was actually the case i~ further evidenced by the results with ethanoh ethanol, the absorption of which is independent of pH, was also much more rapidly absorbed in the small intestine than in the stomach (Table I, Control column}. Other types of deviation from the predictions of the p H partition hypothesis also exist; eg, absorption of the fully ionized quaternary ammonium drugs. Detailed studies of absorption as a function of both dose and time have indicated that something more than passive diffusion plays a role in the absorption of these organic caDigestive Diseases, VoL 15, No. 2 (February 1970)

ACS SYMPOSIUM Table 1. Comparison of Absorptive Capacity of Stomach and Small Intestine in 6-22 Rats Stomach (% absorbed in 1 hr, M ± SO)* Drug

Control

Phenobarbital Pentobarbital Promethazine Ethanol

17.1 23.7 -0.2 37.7

~ ± ± ±

4.7 3.8 3.2 8.6

Treated $ 24.8 30.1 --0.9 46.1

~. -~ ± ±

4.7 4.5t 3.6 4.3[

Small intestine (% absorbed in 10 min, M ± SD)t Control 52.4 54.6 38.2 64.1

=t= 2.3 ± 4.6 ± 6.1 ± 7.5

Treated§ 54.3 57.5 44.3 53.5

~ 5.7 ± 6.0 ± 5.0 =E 5.81[

* Drugs dissolved in 0.01N HCI. t Drugs dissolved in solution, pH 6.0, containing NaCt, KCt, and CaCI2. ++Ethanol present in drug solution in 5% w/v. .~ Ethanol present in drug solution in 1% w/v. !1 Indicates significant difference from controls: p < 0.05. Data taken f r o m Magnussen, M, P. The e f f e c t of ethanol on the gastrointestinal a b s o r p t i o n of drugs in the rat. Acta Pharmacol (Kobenhavn) 26: t30, 1968.

tions. ~, -~. ~-~Another example which m a y be cited is that ol the absorption of dextrom e t h o r p h a n from the rat's stomach. 2a T h e rate of its absorption was found to change with the anion of the salt when p H was held constant (Table 2). G o o d correlation between the absorption rates for the various anions and the surface activity of the salts (measured as surface and interracial tensions) was obtained. T h e data suggested to Fiese and Perrin 2a that the surface-active

Table 2. Absorption of Dextromethorphan Base from Various pH 2.0 Isotonic Buffers

Buffer

Absorption from rat stomach in 3 hr (%)

Hydrochloride Trifluoroacetate Nitrate Trichloroacetate Prefluoropropionate

30.0 31.6 5.2 38.4 40.8

Data taken f r o m Fiese, G., and Perrin, J. H. Further investigations into the absorption of dextrometho orphan from the r a t ' s stomach. J Pharm Sci 58: 599, 1969

Digestive Diseases, I/ol. 15, No. 2. (February 1970)

species, the p r o t o n a t e d d e x t r o m e t h o r p h a n , became attached to an anionic site or a mobile carrier to be transferred across the m e m b r a n e , and that the anion influenced this attachment. T h u s the a b s o r p t i v e process for d e x t r o m e t h o r p h a n appears to be facilitated diffusion rather than simple dip fusion. T h a t the absorption of q u a t e r n a r y amm o n i u m agents m i g h t be influenced by the anion of the salt was postulated earlier by I.evine. 2~ Studies carried out to test this hypothesis showed that the saturated watersoluble fatty acid anions, acetate, propiohate, and butyrate, increased the absorption of the cationic drug~ benzomethamine, a fact also unrelated to the presence of an acidic intestinal e n v i r o n m e n t or to irreversible changes in the gut wall. 2~ Levine did not a t t r i b u t e this absorption enhaficem e n t to i o n - p a i r formation since butyrate also enhanced the absorption of an anionic dye, phenol red. I n the light of the ' m o r e recent study with dextromethorphan,Z~ this effect of the fatty-acid anions may be mediated by a biological effect on the absorptive surface. T h e anion of the salt has been shown also to influence the absorption of 173

LEVINE: GASTROINTESTINALABSORPTION OF DRUGS

the quaternary ammonium drug, pralidoxime (2-PAM); in this case the rate of absorption of the salt of the organic anion, tartrate, was slower than those of salts of inorganic anions and coadministration of tartrate depressed the rate of the iodide salt. 25 Thus deviations from tile usual concepts of factors influencing drug absorption may in themselves be valuable. Once recognized they may prove helpful in elucidating the coexistent variables of the complex biological system and provide the knowledge necessary to alter absorption of drug(s) in the desired direction. Changes in the physicochemical state of the drug in solution, and thus changes in its absorption, also may result from chemical interactions with other drugs or foreign compounds coadministered or present in the gastrointestinal tract. The usual result is a decrease in the amount or rate of the drug's (or drugs') availability for absorption. Most of the unsatisfactory and unintended chemical interactions (incompatibilities) 26. 27 are too well understood and predictable to be reiterated here. The deliberate and intended chemical interactions, such as complexation, used to increase solubility or stability, or to retard absorption when slow sustained release is desired, or to decrease irritancy a n d / o r toxicity are also well known. 7,2s It is sufficient to emphasize that some of the admixtures used in drug preparations frequently produce other than the intended influence on absorption. For example, surface-active agents used as solubilizing adjuncts may produce either facilitation or inhibition of drug absorption, effects which are unrelated to their effect on drug solubility. The inhibitory effect results from complexation-ihe entrapment of the drug molecules within micelles of the surfactant2O--and the enhancing effect from actions of the sur174

factant on the absorbing epithelium, s° There are examples also of complexation which lead to enhanced absorption of the complexed agent: the chelate of iron with ethylenediaminetetraacetic acid (EDTA). ~1. :32 However, this enhanced absorption of iron is without therapeutic value since the stability of the chelate is such that once past the intestinal barrier it does not readily release the cation for utilization.:jr~ That the potential value of complexation as a means of increasing absorption has proved to be of practical significance only as a means of increasing the solubility and stability of drugs, may be largely attributed to the fact that it is an area that has apparently received little attention. Despite the fact that the few investigations carried out thus far have not been encouraging,2~,34 the utili'ty of complex formation to increase the absorption of inefficiently absorbed but highly water-soluble agents deserves further exploration. Interaction with substances normally present in or secreted into the gastrointestinal tract produces changes in the state of the drug which may result in either inhibition or facilitation of absorption. A few examples will serve to illustrate the general problem. The inefficient absorption of quarternary ammonium drugs is attributable, in part, to interaction with mucopolysaccharides of intestinal mucus.21, 35 The elucidation of this mechanism led to the formulation of means of overcoming the problem of mucin-drug binding. 36 This formulation was based on the tested and supported hypothesis that a pharmacologically inactive cationic compound could compete with a pharmacologically active quaternary ammonium drug for mucosal binding sites, thereby making more of the active drug available for absorption.3Z The inefficient absorption of tetracycline has Digestive Diseases, VoL 15, No. 2 (February 1970)

ACS STMPOSIUM

been related to the formation of chelates with polyvalent ions such as calcium and magnesium ~s anti to the formation o~ complexes with proteins in which calcium also plays a r o l e 9 However, the absorption of tetracycline may be enhanced by other normal constituents of the gut, as has been recently demonstrated in the case of the neutral fat, tripahnitin (Table 3).40 T h e enhancing effect of tripahnitin appears to be mediated by removal of calcium. *° Data from other studies tend to support this conclusion since diets containing 25% triolein caused better absorption of calcium than those containing 25% tripalmitin. 41 T h e adverse effect of tripalfifitin on calcium absorption (and its enhancing effect on that of tetracycline) may be due to the formation of calcium soaps as evidenced by increase in fecal volume. 4~ Any drug which is absorbed by the mechanisms of facilitated diffusion or active transport is postulated to interact with carriers or sites which mediate the transfer by temporary combination with the drug. 4 As stated above, the absorption of the protonated d e x t r o m e t h o r p h a n is thought to be mediated in this mannerY 3 Ahhough carriers as such have not been

isolated from membranes, certain identifiable components of membranes have been isolated and shown to be related to the translocation processes of spcific materials. 42 T h e isolation of a fraction of intestinal tissue, the phosphatido-peptide fraction. and the demonstration that the intestinal absorption of various quaternary ammonium drugs may be markedly enhanced in the presence of this phosphatido-peptide may be an example of this phenomenon. 4a--46 While the role of the phosphatido-peptide fi'action in the mechanism of transfer of the cations across the epithelial cell has not been established, the results obtained are compatible with the hypothesis that the cationic agent might be absorbed, in part, through combination with an endogenous anionic carrier. As Levv points out, when such physiologic materials can be isolated, they may be employable as dosage form additives to enhance absorption of the relevant drugs. 7 Factors Related to the N o n a b s o r b i n g Functions and State of Gastrointestinal Tract

Consideration of data presented above (Table 1) on the relative rates of absorp-

Table 3. Effect of Lymph Flow on Absorption in 5 Rats

Drug

Tetracycline* Tetracycline & tripalmitint p-Aminosalicylic acid~ p-Aminosalicylic Acid & Tripalmitint

% of absorbed quantity in lymph (M + SE)

Vol of thoracic duct % of dose absorbed lymph collected from intestine (M + SE) (M ::E SE)

0.26~0.03 0.54i0.05 1.52~0.11

2.76±0.48 5.16~0.73 1.8 ±0.14

18.5~3.2 28.3±2.4 81,1±0.3

2.2 ~0.13

2.62±0.20

83.8±0.6

* Administered intraduodenally in a dose of 20 mg; 2-hr absorption period. t Tripalmitin administered as colloidal solution by stomach tube 2 hr before drug administration. ~tAdministered intraduodenally in a dose of 40 rag; 42 min absorption period. Data taken from DeMarco, T. J., and Levine, R. R. The role of the lymphattcs in the intestinal absorption and distribution of drugs. J Pharmaco! Exp Ther Z69: 142, 1969.

Digestive Diseases,t/ol. 15, No. 2 (February 1970)

175

LEViNE: GASTROINTESTINAL ABSORPTION OF DRUGS

tion from the stomach and intestine of phenobarbital, pentobarbital, promethazine, and ethanol leads directly to the conclusion that the rate of gastric emptying markedly influences the rate at which drugs are absorbed, regardless of whether they are weak acids, weak bases, or undissociated compounds. T h u s the rate of drug absorption and, in turn, the onset of pharmacologic response are directly related to the rate at which drugs are passed from the stomach to the intestine. T h e therapeutic efficacy of a drug which is unstable in gastric fluid is also influenced by its residence time in the stomach. It follows then that any factor which influences the rate of gastric emptying will influence the rate of absorption of an agent administered by the oral ronte. Fortunately this is an area which has received considerable attention and the influences of factors such as food, volume, osmotic pressure and viscosity of contents, age, physical activity, posture, and environmental condition are relatively well understoodY. 4~ T h e pharmacologic activity of certain drugs also is a factor which may influence gastric emptying either by changing the contractile activity of the smooth muscle or by influencing the release of intestinal hormones which modulate gastric activity. Drugs may also induce changes in the secretory functions of the stomach which lead to changes in gastric pH. However, these latter changes have minimal effects on the overall absorption of drugs in general (unless they affect the rate of dissolution or stability of the drugs within the gastric contents). This is true despite the fact that changes in p i t can affect absorption by influencing the rate of gastric emptying. 4s T h e drugs which alter gastric secretion also effect gastric motility and it is the latter effect which predominates. For 176

example, reserpine which increases gastric acid secretion also markedly increases the gastric emptying rate? ~ Changes in the rate of absorption of drugs as well as of foods can be produced by several classes of therapeutic agents which markedly alter gastric motility by their actions on the autonomic nervous system. For example, marked gastric retention and delayed fat absorption have been observed subsequent to the administration of methylatropine nitrate, N0 an action which would be anticipated with any other anticholinergic compound administered in quantities sufficient to depress gastric motility. Codeine and morphine have been found to delay gastric emptying time and depress the absorption of potassium iodide. ~ Other opiate analgesics and antagonists, since they have similar effects on gastric motility, would be expected to similarly depress absorption. Ganglionic blocking drugs may have profound effects on gastric motility; the stomach is relaxed and the emptying of the stomach is greatly inhibited. T h e opposite effect, increase in the rate of stomach emptying, would be produced by cholinergic stimulants. However, the actions of adrenergic amines, such as amphetamine, are not as predictable since their effects are dependent on the preexisting state of activity of the gastrointestinal smooth muscle. Other agents such as iproniazid 52 and chloroquine~a have also been shown to delay gastric emptying by mechanisms as yet undetermined. T h a t various foodstuffs influence the secretion of gastrointestinal hormones and thereby affect gastric motility is well documented. 5a T h a t such an effect may also be produced by drugs has been demonstrated in at least one case. T h e surfactant, dioctyl sodium sulfosuccinate, produces a slowing of gastric emptying and a decrease in the Digestive Diseases, Vol. 15, No. 2. (February-1970)

ACS .SYMPOSIUM

volume and acidity of gastric secretions as a result of the release of enterogastrone upon contact with the duodenal mucosa. ~a T h i s action of dioctyl sodium sulfosucci~ate is not shared by the other surfactants. Intestinal motility is influenced in a m a n n e r analogous to gastric motility by the agents which act on the autonomic nervous system. However, the effect of these agents on rate of absorption from the intestine is opposite to that produced by their action on the stomach. Whereas decreased gastric motility tends to decrease the overall rate of absorption, decreased intestine is opposite to that produced bv rate of absorption, the opposite being tile case for increased motility. Changes in intestinal motility tends to increase rate of absorption, the opposite being the ever, unless there is m a r k e d diarrhea or stasis, or destruction of the agent within the gut. T h e reasons for this last slatement became fully a p p a r e n t when (1) the surface area available for absorption, and (2) the transit time through the various segments of the small intestine are considered together. T h e surface area of the small intestine decreases sharply fro m proximal to distal segments with almost half of the total mucosal area being found in the proximal quarter of the gut. 19,-~° T h u s the proximal part of the small intestine has the greatest capacity for absorption of most drugs as well as for most dietary constituents. ~< 5; Only when an agent is absorbed by a specialized transport process located in the more distal intestine, as for e x a m p l e bile salts, 5s-G° will the absorptive capacity of the more distal segments be greater than that of the more p r o x i m a l ones. However, it must be emphasized that virtually all substances that are absorbed to any extent (with the possible exception of VitaDigestive .Diseases, Vol. 15, No. 2 (February 1970)

min BI_- when })resent in its normally low concentrations) ~1 can be absorbed at any level of the intestinal tract since passive diffusion along electrochemical gradients plays some part in all absorption. Moreover, whether absorption invoh-es a specialized transport mechanism or only a passive process, the crucial factor in determining how much of a drug will be absorbed in a particular segment is not the absorptive capacity of that segment in isolation, but rather how long the material remains in contact with that segment. Since the fastest m o v e m e n t occurs in the duod e n u m and since tile rate of passage of material along the intestine decreases quite rapidly as the material progresses toward the large intestine, the most distal portions of tile small intestine usually become the portions in which the greatest arno,.tnt of d r u g is absorbed. T h e studies of M a r c m and L e n g e m a n n ~~, aa illustrate this rather clearly (Table 4). T h u s prolonging the time a substance is in contact with an area of greater absorptive surface may increase the rate of absorption but has little effect on the total a m o u n t absorbed. Conversely. rapid transit could produce a decrease in tile rate of absorption, but total absorption will be depressed only when the resi-

T a b l e 4.

I n f l u e n c e of Transit T i m e on Absorption

Absorptive capacity (% of dose) G1 tract 'location Duodenum Jejunum Proximal Distal Ileum

Transit Half-Time* Isolated (min) segmentt Total* 6

62

15

18 24 120

65

23

26

62

* Data taken f r o m Marcus and L e n g e m a an62, 6a t Data t a k e n f r o m L e n g e m a n n a n d ComartO"-

177

LEVINE: GASTROINTESTINALABSORPTION OF DRUGS

dence time within the entire small intestine is greatly reduced. At this point, one may justifiably ask whether the oral administration of a drug to a fasted individual is appropriate since entrance of material into the stomach under these conditions usually stimulates rapid transit. Marcus and L e n g e m a n n also f o u n d in studies of absorption from liquid and solid meals that the presence of solids had little or no effect on the transit time through the proximal gut or through the entire small intestine and did not depress the total a m o u n t absorbed. 63 However, the presence of solids almost doubled stomach emptying time. 63 As was indicated earlier, little, if any absorption of basic drugs occurs in the stomach and even for acidic or neutral drugs the intestine represents tile region of greater absorption (Table 1 Control c o h n n n ) . T h u s the motility of the stomach is the rate-limiting factor in absorption regulating tile rate at which a drug is exposed to the area of greatest significance to absorption. W h e n a drug is known to retard gastric emptying or to be unstable in gastric contents, these influences on absorption may be largely overcome and bypassed by appropriate formulation. W h e n drugs disintegrate or dissolve in gastric contents, they pass into the intestine in piecemeal fashion and rarely achieve as high a concentration within the intestinal contents as they initially do in the gastric contents. U n d e r the circumstances where immediate onset of therapeutic activity resulting from gastric absorption is not i m p o r t a n t or where solubilization by gastric conditions is not crucial, the question may be raised whether a great deal of the variability in oral therapy might not be overcome by administering all agents in a form in which rapid dissolution will take place only after a drug 178

has reached the small intestine. The~a the initial dissolution would occur in the area of greater absorptive capacity where changes in motility would be of lesser consequence. Even though changes in intestinal motility have little effect on the total amount of drug absorbed, they may markedly affect therapeutic efficacy through effects on the rate of absorption. Whether an increase in the rate of drug absorption produces systemic levels high enough to lead to unwanted effects or a decrease in rate of absorption produces levels too low to have therapeutic effect will be determined by other rates--those of metabolism and excretion. In view of the great absorptive capacity of the intestine, the blood flow of the mucosa and particularly of the villi are of vital importance since blood constitutes tile major vehicle for removal of absorbed material. Although estimates of the amount of the total intestinal blood flow that is diverted to the mucosa range from 176' to 60%, '~° it seems that the intestinal mucosa is somewhat overperfnsed in relation to its nutritional demands when compared with the kidney. G'~This large blood supply makes it unlikely that decreased flow will produce any significant decrease in rate of absorption of materials aborbed by passive diffusion. Removal from the lamina propria does not become a rate-limiting factor until there is marked reduction in blood flow for a period of time sufficient to produce structural damage, 66 T h e few reported studies of the interrelationship of intestinal blood flow and absorption of nutrients 6n, 67 and drugs ~s indicate that a decrease in the blood supply to the villus may be expected to retard intestinal absorption of some actively transported drugs and to have little effect on others. Digestive Diseases, Vol. 15, No. 2 (February 1970)

ACS SYMPOSIUM

In the stomach, in contrast to the intestine, changes in blood flow can influence absorption of agents transferred even by passive diffusion. Thus, not only the size of the relative absorbing surface but the relative quantity of blood flow may be a rate-limiting factor in gastric absorption. This is amply illustrated by the studies of Magnussen ~s of the effect of ethanol on barbi.turate absorption (Table 1, T r e a t e d column). Ethanol administered together with the drugs (or present in the animal in a steady concentration during the time of absorption) was found capable of significantly enhancing barbittu'ate absorption from the stomach but to be without influence on absorption from the small intestine. T h e enhancing effect of ethanol on barbiturate absorption was m e d i a t e d through an increased blood supply to the gastric mucosa. T h e lack of effect of ethanol on absorption from the intestine was attributed to the fact that relatively small variations in blood supply could not improve already optimal conditions for absorption. While changes in the rate of blood flow through the intestinal villus have little influence on the a m o u n t of absorbed material removed via this route, changes in the rate of flow within the other circulatory system, the lymphatics, do , influence the amount of absorbed material removed via lymph. Although the lymphatic route is rarely even considered where drug absorption is concerned, the fact is that any substance which is transferred through the intestinal epithelium into the lamina propria has access to both blood and lymph capillaries. 4° Also, material which leaves the intestine via lymph gains access to the general blood circulation without first passing through the liver, thereby averting potential metabolism prior to distribution. Digestive Diseases, VoL 15, No. 2 (february 1970)

Ordinarily, a much greater proportion of a small molecular species is absorbed via the blood than via the lymph because the rate of blood flow is several h u n d r e d times that of lymph flow. ~0 For example, De Marco and Levine have shown that only a small percentage of a well absorbed drug, p-aminosalicylic acid, or all inefficiently absorbed drug, tetracycline, was absorbed via lymph under normal conditions. 40 However, the amounts of these agents carried away ill the lymph was doubled when lymph flow was stimulated by the administration of tripahnitin (Table 3).4° While this increase in the a m o u n t of absorption via lymph would have little therapeutic consequence in the case of the two drugs just mentioned, such an increase might have relevance to the effectiveness of oral therapy with agents which are only slightly absorbed at best and which are susceptible to hepatic metabolism. In this respect it is well to remember that the lymphatic route is the major pathway for absorption of large molecules, such as cholesterol, 70 protein, 7x and fatty acdis. 7-~ T h e normal, physiologic state of the gastrointestinal tract is one in which the microbial flora are an integral part. Almost as soon as bacteria were discovered, the question was raised as to tire physiologic and clinical significance of the microflora present in healthy animals. Pasteur, reasoning teleologically, felt that since bacteria were normally present in a p p a r e n t l y healthy men, they were ahnost certainly serving a useful purpose and that animal life without microorganisms would not be possible. T h a t Pasteur's view is not correct in the limited sense stated is evidenced by the fact that several species of animals have now been raised and bred germfree. But, in the broader sense, there is evidence that such a view has some validity. T h e organs 179

LEVINE: GASTROINTESTINALABSORPTIONOF DRUGS

and tissues not usually in contact with bacteria, the organs of the internal environment, are essentially similar in germ free and conventional animals. On the other band, the organs which stand between the constancy of the internal milieu and the inconstancy o[ the external environment and which act as buffers have been shown to differ in the two types of animals. In particular, the gastrointestinal tract, exposed to an environment teeming with microorganisms has been found to be markedh' altered by those microbes, not only morphologically 7a but also functionally with respect to rates of absorption of nutrients.74, 7a ttowever, there have not been any studies carried out to determine the effects of either the presence or the drug-induced absence of the microflora on drug absorption, a h h o u g h the enzymatic activity of the microflora has been shown to influence the a m o u n t of drug available [or absorptionS< 7z Studies with gnotobiotic and muhicontaminated animals have shown that interactions between the various groups of bacteria in the GI tract are likely also to have a bearing upon the activities of an)' single microbial speciesY s Therefore, in the conventional animal the use of antimicrobial agents may affect the absorptive functions of the intestine by altering the n u m b e r as well as the balance of its microbial population. It is interesting to note that following oral administration of various antibiotic combinations antibacterial activity has been shown to be unpredictable and frequently not as good as the most effective agent in the particular combinationY 9's0 'Whether altmations in absorption play some role in this p h e n o m e n o n also has not been investigated. Certainly studies to determine the effect on drug absorption of drug-induced inhibition of the normal intestinal microflora need to be carried out. 180

Factors Related to the Metabolic Activities of the Epithelium

T h e metabolic activities of the absorptive cell itself may influence absorption in both rate and a m o u n t in several ways. T h e fundamental processes responsible for maintaining the structural organization of the cell and the integrity of the cell membrane are ordinarily functioning at optimal levels ~ so that only inhibitory effects are detectable. W h e n inhibition of these fundamental processes leads to a disruption of the structural integrity of the cell or celt membrane, then the question of alterations in absorptive capacity become irrelevant. st In the absence of structural damage, however, inhibition of these general metabolic activities may result in nonspecific inhibition of drug absorption if absorption proceeds by a specialized process requiring energy over and above that needed to maintain cellular organization. For example, benzmalecene inhibits the transport of taurocholate, glucose, and tyrosine by inhibiting enzymes of tile Krebs cycle,s2 On tile other hand, specific inhibition of absorption of a single drug may be produced by another drug which inhibits or "competes for a specific transport mechanism: the foreign pyrimidines, 5-fluorouracil and 5-bromouracil, compete with each other, sa and the absorption of tile cardiotonic steroid, convallatoxin, is inhibited by hydrocortisone, s4 Rather recently, it has also been recognized that the intestinal mucosa can influence drug absorption through its capacity for carrying out a wide variety of drug biotransformations per se. sa-ss T h u s despite the fact that the rate and amount of drug transferred from the lumen of the intestine may remain unchanged, the other part of tile total absorptive process--the entry of the drug into the circulation--may Digestive D~seases,Vol. 15, No. 2 (February 1970)

ACS SYMPOSIUM

be markedly altered in terms o f therapeutic value by previous metabolism within the intestinal mucosa. In turn, the influence of. these metabolic processes may be altered by the rate at which drug is presented to them a n d / o r by the presence of other agents. Studies have indicated that at least the intestinal glucuronide conjugating system s~ and benzpyrene hydrolase ~9 may be increased by metabolic inducers. Since chronic oral therapy with one or more agents is so common, there is great likelihood of other drug interactions of this type. Ordinarily, the amount of increased drugmetabolizing activity in the intestine would be of little consequence to tlle total a m o u n t of drug available in the body in view of the much greater influence of stimulated liver activity. However, for drugs given in small doses, or for those slowly absorbed or slowly released from pharmaceutical preparations, the effect of intestinal biotransformation may be of major significance. T h e metabolic activities of the intestinal epithelium concerned with the maintenance of structural and functional activity are probably of lesser magnitude than those activities concerned with the massive bidirectional fluxes of water and electrolytes across the intestinal ceils. T h e volume of the daily circulation of water and electrolytes secreted into the gastrointestinal tract and absorbed from it in order to maintain osmotic conditions, far exceeds the dietary intake. For man the quantities of water and salt entering daily from gastrointestinal secretions have been estimated at 7000 cc and 595 m M respectively. 9° T h e weight of present evidence clearly indicates that the translocation of sodiumion from the intestinal lumen across the epithelium is an active process which is closely coupled to the absorption of other solutes such as gh~cose and amino acids, :9igestive Diseases, Vol. !5, .No. 2. (February 1970)

as well as to the absorption of water21,9-" A h h o u g h much remains to be investigated and elucidated concerning the relationship and interdependence of intestinal absorption of water, sodium-ion and other solutes normally present in the intestine, the data already available are sufficient to ask the question whether the absorption of water a n d / o r sodium-ion also influences the absorption of drugs. We have undertaken such investigations only recently; thus, the studies a r e just in the preliminary stages. However, the restilts of initial studies are so encouraging that presentation of even partial results at this time is warranted in view of the point tinder discussion. These preliminary studies have shown that the absorption of tetracycline from the rat jejunum, in vivo, can be significantly increased when the dissoh'ing medium is isotonic NaC1 rather than water (Table 5). This stimulation of tetracycline absorption b v NaC1 was abolished when rats were pretreated with ouabain, a drug considered to be a specific

Table 5. Absorption of Tetracycline (Tet) in the Rat in the Presence of Various Solutions

Solution 1 ml water 4 ml water 1 ml 0.9% NaCI 4 mt 0.9% NaCl 4 ml 0.9% KCl 4 rn 0.9% NaCI & ouabain++

No of rats 7 9 10 6 9 4

Tet absorbed in 3 hr (M% ± SD) 26.2 ± 32.2 ± 35.9 ± 48.5 ± 39.3 ±

2.7* 2.9+ 2.6 2.7 3.1f

32.4 ± 2.5f

Dose = 1 mg Tet in jejunal loops. * Significantly lower than 1 ml 0.9% NaCt; p < 0.05. t Significantly lower than 4 ml 0.9% NaCl; p <

0.05. Dose of ouabain, 10 rng/kg iv, administered ~3 pqn before beginning of absorption period;

181

LEVINE: GASTROINTESTINALABSORPTIONOF DRUGS inhibitor of sodium-ion transport. ~z Obviously, these studies are continuing and a complete report, hopefully, will be available shortly. ~3 T h e elucidation of a possible role of sodium-ion in the absorption of certain drugs would not only be important for the drugs concerned but also for many therapeutic situations since a large n u m b e r of agents in clinical use have been shown to inhibit the transport of sodium-ion, Among these are the cardiac glycosides, '~-° various diuretics, 94 hormones, 9'5 and cathar. tics. 96 Consideration of the large daily circulation of fluid within the intestinal tract also provokes the question of whether the flow of solvent itself, so-called solvent-drag, influences the absorption of drugs. T h e r e is some indication that flow of solvent does influence the absorption of urea, creatine, and sorbitol, 97 as well as NaC1, :~s but studies have not been carried out to provide any data concerning the influence of solvent-drag on drug absorption. This despite the fact that oral preparations are usually taken with water and that the quantity imbibed may have some influence on absorption or variability of absorption. We have recently begun such studies, but the data are too few as yet to make any statements about their import. Factors Related to the Structure of the Absorbing Surface--the Intestinal Epithelium A change in structure of the absorbing surface may be simply a change in the physical properties of the barrier, such as surface tension or charge, or an alteration in its architecture or morphology which may be either transitory or destructive. If a c o m p o u n d causes a breakdown in the structure of the cell membrane, one of the main prerequisites of transport, the barrier 182

to diffusion, is lost. T h e n , as stated earlier, the question of facilitation or inhibition of the absorption of a second compound becomes a moot point. T h e importance of tile integrity of the cell membrane as a condition for the study of intestinal absorption cannot be overemphasized since so often it has been overlooked or ignored. For the most part, studies of intestinal absorption of drugs and other solutes have not included simultaneous histologic evaluation, even when the agents used have been assumed to exert their effect on the biologic membrane. In fact, the interpretation of the results of many of the studies using the everted intestinal sac technic should be re-evaluated in the light of a recent study indicating that this preparation begins to lose structural integrity within 10-15 rain after incubation in appropriate media even in the absence of drugs 9'., T h a t the absorptive capacity and function of the intestine is dependent on its morphology is well documented by the studies of various malabsorption syndromes ~°° and by the studies in germ free animals. However, some agents can apparently influence drug absorption by producing reversible structural alterations of a transitory nature. Certain anionic or nonionic surfacfants appear to enhance ahsorption of various drugs by altering the physical properties of the barrier, z and chelating agents increase the absorption of various drugs by widening of intercellular channels. ~ol It becomes obvious though, that the influence on absorption produced by changes in the structure of the absorbing cell or surface are mainly associated with abnormal or pathologic conditions, while changes in the physiologic function or state of the intestine may influence drug absorption as a consequence of completely normal conditions. Digestive Diseases,Vol. 15, No. 2 (Februaly1970)

AC$ SYMPOSIUM CONCLUDING REMARKS T h e use of conceptually simple systems as models may be justified fully only when the necessary data provided by this approach are used to direct attacks on problems of a higher order of biologic complexity. Understanding of pharmacodynamic mechanisms has been achieved by successful attempts to isolate even simpler and more fundamental components of living systems. In contrast, the understanding of pharmacokinetic problems is successful only to the extent that more and more complex variables are incorporated so that the model is applicable and relevant to the real situation. Therefore, data of absorption whether descriptive in nature or definitive as to cellular mechanisms, need ultimately to be reassessed under conditions that coexist during the actu,~l process of absorption if we are to exploit our knowledge and continue to profitably pursue our original objective--ie, to learn how to make the oral route a more effective as well as a more consistently reproducible means of drug administration. Recent investigations, while again directing attention to the complex nature of intestinal absorption, have clarified certain points in the absorption of drugs as well as of normal dietary constituents. First of all, there is a unity in the mechanisms available for absorption. T h e process of diffusion for drugs is not different from that for nutrients, and the variances attributed to chemical structure are far less than the variance attributable to the biologic properties of the tissues as a whole. Where an active transport process has been identified for drugs it has confirmed that drugs use pre-existent mechanisms such as those for normal body constituents. Moreover, the influences on absorption of the physiologic factors of gastrointestinal p H and motility, Digest~ Diseases, VoL 15, No. 2 (Februar/ 1970)

having been rather thoroughly investigated, are relatively well understood and shown to be the same for both drugs and nutrients. But the influences exerted by: (1) tile existence and magnitude of tile fluid and electrolyte fluxes of the gastrointestinal tract; (2) the coupling of ion transport to other transport processes; and (3) the microflora, either normal or pathologic, have received only cursory attention with respect to drug absorption, even though they are being intensively investigated in the case of normal dietary constituents. T h e influences of a n u m b e r of drugs on the absorption of various ions, sugars, amino acids, and lipids also have been recently defined. Yet again little information is available concerning the effect of a chronically or coadministered drug on the absorption of another therapeutic agent. This despite the [act that drug combination therapy is extremely common. T h u s , the provinces of questions to be asked and answered about drug absorptoin have been defined, and previous work done in the area has provided the preliminary but necessary data to attack the problems of absorption with an holistic approach. REFERENCES 1. BRODIE, B. B., and HOGBEN,C. A. M. Some physico-chemical factors in drug action. ] Pharrn Pharmacol 9:345, 1957.

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Meal 4:627, 1957. 39. KOHN, K. W. Mediation of divalent metal ions in the binding of tetracycline to macromolecules. Nature (London) 191:1156, 1961. 40. DE ~IARCO,T. J., and LEViNE, R. R. T h e role of the lymphatics in the intestinal absorption and distribution of drugs, f Pharrnacol Exp Ther 169:142, 1969. 41. TADAYON,B., and LL:TWAa, L Effects of dietary triolein, tripalmitin and I-phenylalanine on calcium absorption in the rat. Proc Soc Exp Blot Med 130:978, 1969.

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Digestive Diseases, VoL 15, No. 2 (FebrOary 1970)