`
`DRUG ABSORPTION, ACTION, AND DISPOSITION
`
`705
`
`The Nature of Receptor Groups
`and Models of Receptors
`
`A receptor groupis that portionof the receptor molecule
`with which an agonist acts and whichis vital to the function
`of the receptor. Studies of receptor group composition and
`configuration are too complex for the purposes ofthis text;
`consequently, only a brief sketch will be made here toorient
`the reader to the nature of the approach.
`From the chemical configuration and reactivity of ago-
`nists and antagonists, certain deductions can be made about
`the structure of a receptor group. For example, all bighly
`active agonists of muscarinic receptors are cations at physio-
`logical pH. This suggests that the receptor group contains
`an anionic group and that the force of attractionis electro-
`static, at Jeast in part, which agrees with thermodynamic
`data. That van der Waals forces (especially Heitler-London
`fluctuation forces) may also make an important contribution
`to binding is suggested by the requirement for N-methyl
`groups and by the low butdefinite activity of the nonioniz-
`able quaternary carbon analogof acetylcholine, 3,3-dimeth-
`ylbutyl acetate. This establishes a requirement foran auxil-
`iary structure close to the anionicsite. Studies of the contri-
`bution to activity of ester and carbonyl oxygen among ana-
`logs of acetylcholine,
`intramolecular distances and the
`stereospecificity of various isomers and conformers have
`indicated a partial cationic (proton donor} site between 2.5
`and 4 A and a region of high electronic density (electron
`donor} between 5 and 7 A from the anionic site. This is
`similar to the wayin which the active site of acetylchclines-
`terase was mapped(see page 427, and Figs 25-44, 45 and 46).
`The structure-activity relationships among competitive
`inhibitors also must be consistent with any modelof a recep-
`tor. However, bindingsites additionalto the receptor group
`can he involved, and results are frequently more difficult to
`interpret than those with agonists. Nevertheless, studies
`with antagonists have made a substantia! contribution to
`receptor group analysis.
`‘There is considerable interest in
`antagonists that combine irreversibly with the receptor,
`since such drugs offer a way of marking (affinity labeling)
`
`the receptor for isolation and for identification ofthe recep-
`lor group.
`Since receptors for auLonomic agonists are embedded in
`the cell membrane, they have beendifficult to isolate with-
`oul inactivation. Several laboratories have succeeded in
`isolating proteins, the chemical properties of which are con-
`sistent with those expected of various receptors. Receptors
`far steroid hormones have been easier to isolate, and some
`have been characterized relatively well. Further details of
`drug-receptorinteractions and the nature of receptors can
`be found in the works on receptors and molecular pharma-
`cology.
`Up- and Down-Regulaition—in many receptor-effector
`systems,
`if there is a paucity of agonist, the system will
`respond by increasing the responsiveness, number ofrecep-
`tors on the effector membrane or numberof coupling pro-
`teins or enzymes in the effector system. This is known as
`up-regulation.
`In adrenergic systems, sympathetic dener-
`vation has been shawnto increase the numberofpast-synap-
`tic B-adrenoreceptors al some junctions and the availability
`of nucleotide-binding proteinunits and/or adenylate cyclase
`molecules at others. Hyperthyroid activity also increases
`the number of 8-adrenoreceptors in heart muscle, which
`explains the excessive heart rate, Denervation of skeletal
`muscle causes a great multiplication of what is narmaily a
`minor type of nicotinic receptor, and the new receptors
`spread across the entire myocyte membrane. Prolonged
`blockade of receptors by antagonists also may cause up-
`regulation. The abrupt discontinuation of treatment, such
`that drug levels fall faster than re-regulation, may be fol-
`jowed by excessive activity, eg, in pernicious tachycardia and
`angina pectoris from abrupt withdrawal of propranolol.
`Excessive agonism will lead Lo a decrease in Lhe numberof
`receptors or in stimulus-response coupling. This is one
`cause of lachypbylaxis or tolerance, such as occurs to the
`bronchodilator effects of 8-adrenoreceptor agonists.
`Abrupt withdrawal may result in poor residual function orin
`rebound effects, depending uponthe typeofeffect caused by
`the agonist.
`JExcessive agonism also may cause desensitiza-
`tion by agonist-induced changes in receptor conformation to
`inactive, slowly reconformable states.
`
`Mechanism of Drug Action
`etc. Such terms describe only the effect and nat the action,
`Any metabolic or physiological function provides a poten-
`tial mechanism ofaction of adrug.
`‘The term mechanism of
`and they have no bearing upon whether the drug augments
`receptor function or diminishes it.
`In biological systems,
`action has been employed in a numberof ways.
`In the past
`it was often the habit to confuse the site, or locus of action,
`positive and negative modulation and feedback occur at
`with the mechanism of action. For example, the mechanisin
`every level, the organ as well as the subcellwar. Thus, an
`agonist to a negative modulator may beable to bring about
`of the hypotensive action of tetraethylammoniumionorigi-
`the same effect as an antagonist to a positive modulator.
`It
`nally was described as thal of ganglionic blockade, which did
`is possible for an antagonist or inhibitortoelicit an excitato-
`nothing more than identify the anatomical structure upon
`ry effect. An example is the convulsant action of strych-
`which the drug acted.
`In a general sense, this was a partial
`elucidation of the mechanism of action, if mechanismis used
`nine, which results from its anlagonismofglycine, an impor-
`in the mechanical sense of the entire linkage between the
`tant mediator of postsynaptic inhibition in the central ner-
`vous system. Conversely, il is possible for an agonist to
`input and output of a machine. However, there has heen a
`elicit an inhibitory effect. An example is the reflex brady-
`gradual narrowing of the definition of mechanism ofaction
`cardia that results from the stimulant action of veratrum
`to be restricted to only the first event in the action~effeet
`alkaloids on chemoreceptors in the left ventricle.
`sequence, that is, only to the alteration of receptor function
`Because of the central role enzymes play mmcellular fimc-
`by the drug.
`In this sense, the mechanism of action of
`tion, it is net surprising that thoughts about the mechanism
`tetraethylammonium is defined more appropriately as that
`of action of drugs has focused largely upon enzymes. Ago-
`of competition with acetylcholine for nicotine cholinergic
`nist drugs conceivably could serve as substrates, cofactors ar
`receptors on the postsynaptic ganglion cell membrane, even
`activators. At the present time, no drug is knowndefinitely
`though the alteration in receptor function is not defined.
`to exertits action as a substrate or as a cofactor, exclusive of
`The ultimate mechanism of action is known for only a few
`vitamins and known nutrients. However, at least three
`drugs.
`classes of drugs are known andseveral are suspected to work
`It ig customary to speak of a drug as a stimulant or a
`through the activation of enzymes.
`depressant, of the action as being excitatory or inhibitory,
`
`
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`
`
`706
`
`GHAPTER 35
`
`"The most. notable example of enzyme activationis that. of
`epinephrine and similar #-adrenoreceptor agonists, which
`activate adenyl cyclase (o increase the production of 3',5"-
`eyclic adenylie acid (cyclic AMP; cAMP).
`‘The metabolic
`and cardiac effects of catecholamines are attributable,
`in
`part, bo the increment in cAMP. One modulatoraf adeny]
`cyclase is the B-adrenergic receptor.
`‘The §-adrenoreceptor
`is coupled to adenylate cyclase through a regulatery protein
`that binds GDP and GTP (G-protein). When GDP is
`present, the agonist-receptor complex is associated with the
`regulatory protein. G'TP causes transferof the regulatory
`prolein to adenylate cyclase and dissociation ofthe #-adre-
`noreceptor. Glucagonalso owesits hyperglycemic action to
`activation of hepatic adenylate cyclase. A numberof other
`agonists also activate adenylate cyclase.
`‘There is, thus, the
`interesting phenomenon of one enzyme, adeny! cyclase, be-
`ing activated by numerous chemicaily unrelated drags.
`Since 6-adrenergic-blocking agents do not antagonize gluca-
`zon, it is obvious that glucagan works upona differentrecep -
`tor than does epinephrine.
`Thus, cAMPactivates protein kinases that increase the
`activity of phosphorylase, actomyosin, the sequestration of
`calcium by the sarcoplasmic reticulum and calcium chan-
`nels. Therefore, a brief’ activation of the f-adrenoreceptor
`sets in motion a cascade of events that greatly amplify the
`signal. Kinases also participale in down-regulation and de-
`sensitization.
`Other important enzymes coupled to receptors are gua-
`nylate cyclase and phospholipases A and C, which are in-
`volved with membrane fluidity and caleium channels, re-
`spectively.
`Many drugsareinhibitors of enzymes. Whenthe drugis a
`competitive inhibitor of a natural endogenous substrate of
`the enzyme,itis called an antimetabalite (see also page 431).
`Kéxamples of antimetabolites are sulfonamides, which com-
`pete with para-aminobengoic acid and, thus, interfere with
`its incorporation into dihydrofolic acid and methotrexate,
`which competes with folic acid for dihydrofolate reductase
`and, Unus, interferes with the formation offolinic acid.
`It
`might seem that anticholinesterases are also antimetabo-
`lites, although they are never placed intothat classification.
`The reasonis that the products of cholinesterase--acetyleha-
`line interaction do net subserve important metabolic func-
`tions, as do folic andfolinic acids, so that the organism is not
`deprived of an important metabolite by the action ofthe
`cholinesterase inhibitors.
`Some drugs are competitive inhibitors of enzyme systems
`whose natural function appears not to produce useful me-
`tabotites but to rid the body of foreign substances.
`Inhibi-
`tors of the hepatic microsomes and prebenecid fall into this
`category; the hepatic microsomes doperforma few biotrans-
`formations on endogenous substrates, but the renal tubular
`anion transport system does nat appear to be required to
`eliminate any important endogenous substances,
`Since neither the hepatic microsomes nor the tubular an-
`ion Cransport system seems to be involved in response sys-
`tems, inhibitors of these enzyme systems are antagonists
`without corresponding agonists.
`Indeed, even natural en-
`dogenous substrates of enzymes ave rarely considered to be
`agonists.
`Noncompetitive enzyme inhibitors among drugs alsoare
`known. Examples are cyanide, fluoride, disulfiram and car-
`diac glycosides. When enzyme inhibition brings about a
`positive response—ep,
`the cholinergic effects of the anti-
`
`cholinesterases or the effects of diazoxide consequent to
`inhibition of phosphodiesterase---Lhe drug appears to be an
`agonist. Yet, there can be ne competitive antagonistto such
`an inhibitor, since the competitor to the drug is more sub-
`atrate, to whichthe effect. of the drug is actuallyattributable.
`Acetylcholine increases the permeability ofthe subsynap-
`
`tic membraneto cations and the heart muscle membrane to
`potassium,
`‘The mechanismis thoughtgenerally to invalve
`a change in conformation of a protein constituent of the
`potassium channel, so that pore size ar permeability con-
`slantis affected. The muscarinic receptoris coupled to the
`potassium channel through a G-pretein. Other autonomic
`ayonists also are known10 alter the permeability to ions, in
`part through activation of adenyl cyclase, guany) cyclase,
`phospholipase-e or other enzymes, Many drugs and toxins
`
`act through alterations inthe structural and physical prop-
`erties of membranes.
`‘To the extent that some ofsuch sub-
`stances may disperse themselves generally throughout the
`lipid phase of the membrane rather than to combine with
`special chemical entities, no definite receptors for such
`drags can be said to exist.
`The mechanism ofaction ofcertain drugs, especially auto-
`nomic drugs, often is stated to be mimicry of a natural
`neurohumoror hormone. Thus, methacholine mimics ace-
`tylcholine ag an agonist. This does not define the mecha-
`nismof action, unless the mechanismof action of the natural
`substanceis known,
`Mimicry usually occurs because of a structural similarity
`between the natural substance and the mimetic drug. Mim-
`icry in agonist fanctions is easy to demonstrate, but the site
`af action may notalways be mimicry of the natural agonist al
`iis receptor bat rather at anallosteric site on a receptoror at
`its storage site to release the natural agonist.
`lixamples of mimetics that act by release of the natural
`inediatorare indirectly acting sympathamimetics suchas d-
`amphetamine, mephentermine, ephedrine (in part), tyra-
`mine and ethers, which are now known to act by displacing
`norepinephrine from storage sites within the adrenergic
`neuron. Mauyof such indirectly acting sympathomimetics
`lack a direct action on the adrenergic receptor, although
`some, Hke ephedrine, ael both upon the receptor and the
`starage complex. Another mimetic by a release mechanism
`is carbachol, which promotes the presynaptic discharge of
`acetylcholine.
`In these examples, there is a close structural similarity
`between Lhe mimetic and the released mediator.
`In the case
`of many releasers of histamine (such as tubocurarine, poly-
`myxin or morphine}, no close chemical relationship exists
`between the releaser and the released.
`Jn such instances,
`release has heen explained by activation of receptors on the
`mast-cell membrane which promote exocytosis of the hista-
`ming-containing granules, by an influx of calcium andacti-
`vation of microtubules, all of which may be involved in
`moving the granules out of the mast. cell.
`Structural similarity also may aid mimicry by promoting
`chemical combination with an enzyme of destruction or
`some other means of disposition. For example, metara-
`minol, amphetamine, ete inhibit membrane iranspart into
`the neuron and, hence, inkibit the neuronal recapture of
`released norepinephrine. Consequently, the extraneuronal
`concentration of norepinephrine in the nearbyregion of the
`receptors does not drop as rapidlyas in the absence of the
`mimetic, and the action of the mediatoris sustained.
`Someinhibitors of the enzymes ofthe destruction of medi-
`ators are structurally similar enough to the mediator to have
`some agonist action.
`‘This is true of neostigmine, which has
`a direet stimulant action on nicoLinic receptors in addition to
`ilg anticholinesterase action.
`In contrast, the anticholines-
`lerase, physostigmine, has some antagonist actions on cho-
`linergic receptors andalso aneffect. to interfere with acetyl-
`choline synthesis.
`‘The above multiple actions come about because all the
`structures that interact with a small molecule mediator (the
`receptor, synthesizing enzyme, destructive enzyme, storage
`molecule, membrane transport carrier) must have some
`commonstructural features andaifinities. A drug thatre-
`
`
`
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`

`acts with one of these molecules has a distinct probabilityof
`interacting with another.
`The recognitionof the critical role of fons in the function
`of membranes, the excitability of cells and the activity of
`many enzymes has generated a renewed interest in fons in
`the mechanism of action of certain drugs.
`‘The inorganic
`ions, some of which are used as drugs, lend themselves auto-
`matically toa discussion of ionic mechanisms.
`‘The repairof
`electrolyte deficiencies by replacement therapy warrants no
`further comment here. Some nonphysiological ions act as
`imperfect impersonators of physiological ions; lithium part-
`ly substitutes for sodium, bromide for chloride and thiocya-
`nate for iodide, and each may owe its pharmacological ac-
`tion, in part, to a sluggish mobility through membrane chan-
`nels, through which their sister jong normally pass readily
`when traffic is nol impeded by “slowly moving vehicles.”
`Jodide has aneffect. to Increase the penetrance of drugs into
`caseous and necrotic areas, to aid in the resolution of gum-
`matous lesions, to reduce the viscosity of mucous secretions
`and other addeffects; it is Chought to do so by increasing the
`hydration of collagen and mucoproteins by a poorly under-
`stood mechanism. The transition elements and heavy met-
`als have in common the ability to form complexes with a
`variety of physiologically active substances, particularly the
`active centers of many enzymes, Chelation and otherLypes
`of complexation are the mechanisms of action of several
`drugs used to treal heavy-metal intoxication, diseases that
`invelve abnormal body burdens or plasma levels of heavy
`metals and hypercaicemia. Chelates and chelation are dis-
`cussed in more detail in Chapter 14.
`There js muchinterest in the effects of drugs on ion move-
`ments, Cardiac glycosides are known toinhibit an ATPase
`
`DRUG ABSORPTION, ACTION, AND DISPOSITION
`
`vor
`
`involved in the membranetransport. of sodium andseveral
`other substances, which indirectly causes an increase in in-
`tracellular caleium content.
`In part, the mechanisms of
`actionof local anesthetics, quinidine and various otherdrugs
`also are speculated to involve calcium movements.
`In the
`past. decade there has appeared a whole newclass of drugs,
`the calcium chansel blockers.
`Concomitant with the development of molecular biology
`was the appreciation that drugs act through nuclear and
`extranuciear genelic mechanisms. Nitrogen mustards
`have long been knowntointerfere with the replication of
`DNA. Streptomycin, kanamycin, neomycin and gentamicin
`cause misreading by the ribosomes ofthe code incorperated
`into messenger RNA; tetracyclines, erythromycin and chior-
`amphenicolinhibit the synthesis of protein at the ribosomes;
`and chioroguine, novobiocin and colchicine inhibit DNA
`polymerase, Other drugs induce the productionof enzymes;
`aldosterone appears Lo act by inducing the synthesis of Lhe
`enzyme, membrane ATPase, necessary to sodium transport.
`In general, steroid hormones combine with a cytosolic recep-
`tor, the complex of which is processed and translocated ta
`the chromatin, where gene expression is altered. Many
`drags induce ene or more of the hepatic and extrahepatic
`cylochrome P-450 enzymes.
`A number of drugs have simple mechanisms that do not
`involve an action at the cellular level. Examples are bulk
`and saline catharties, osmotic diuretics and cholestyramine.
`Although such drugs usually do not generate much excite-
`ment among pharmacologists, they do serve as a reminderof
`the many avenues through which a mechanism ofaction may
`be expressed.
`‘Throughout the various chapters of Part 6,
`specific mechanisms of action may be mentioned.
`
`Absorption, Distribution and Excretion
`faces, like icebergs; ie, mucb of the protein is below the
`No matter by which route a drug is administered it must.
`surface.
`In Fig 35-8 the lipid layers are represented as a
`pass through several to manybiological membranes during
`somewhat orderly, closely packed lamellar array of phospho-
`the processes of absorption, distribution, biotransformation
`and elimination. Since membranes are traversed inall of
`lipid molecules associated tail-to-tail, each “tail” being an
`alkyl chain or steroid group and the “heads” being polar
`these events, the subject of this section will begin with a brief
`groups, inchiding the glycerate moieties, with their polar
`descriptionof biological membranes and membrane process-
`ether and carhonyl oxygens and phosphate with attached
`es andthe relationship of the physiochemical properties ofa
`polar groups.
`Inreality, the lamellar portion is probably nat
`drug molecule to penetrance and transport.
`
`~~ é yy EXTAAGELLULAR
`“a
`
`Gi"
`
`Structure and Properties of Membranes
`
`The concept that a membrane surrounds eachcell arose
`shortly after the cellujar nature of tissue was discovered.
`The hiclogical and physiochemical properties of cells
`seemed in accord with this view.
`in the past, from time to
`time, the actual existence of the membrane has been ques-
`tioned by brilliant men, and ingenious explanations have
`been advanced to explaincellular integrity and the osmotic
`and clectrophysiological properties of cells. Microchemical,
`x-ray diffraction, electron microscopic, nuclear magnetic
`resonance, electron spin resonance and otherinvestigations
`have proved both the existence and nature of the plasma,
`mitochondrial, nuclear and othercell membranes.
`‘The de-
`scription of the plasma membranethatfollowsis much over-
`simplified, but. it will suffice to provide a background for an
`understanding of penetrance into and through membranes.
`Structure and Composition—The cell membrane has
`been described as a “mayonnaise sandwich,” in which a
`bimolecular layer of lipid material is entrained between two
`parallel] monomolecular layers of protein. However, the
`protein does not make continuous layers, like the bread ina
`sandwich, but rather is sporadically scattered over the sur-
`
`“, _# oe.
`
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`_ INTRACELLULAR
`Fig 35-8.
`Simpiifiod cross section of a cel! membrane {components
`are not to scale), The lipid interior of the lamellar portion of the
`membrane consists of various phospholipids, fatty acids, cholesterol
`and othersteroids.
`lons are indicated tn orderto illustrate differences
`in size ralative to he channel. Pr: protein, Sus
`sugar.
`
`
`
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`

` 708
`
`Diffusion and Transport
`
`where @ is the net quantily of drug transferred across the
`membrane,¢ is time, C, is the concentration on one side and
`Con theother, x is the thickness of the membrane,A is the
`area and 2 is the diffusion coefficient, related to permeabili-
`ty. The equation is more nearly correct if chemical activi-
`ties are used instead of concentrations. Since a biological
`membrane is patchy, with pores of different sizes and proba-
`bly with varying thickness and composition, both D and x
`probahly vary Crom spot to spol. Nevertheless, some mean
`values can be assumed.
`It is customary to combine the membrane factors into a
`single constant,calleda permeability constantorcoefficient,
`P,so that P = B/x, Ain Eq 5 having unit value.
`‘The rate of
`net transport(diffusion) across the membrane then becomes
`
`eeemeeeeen HR
`
`CHAPTER 35
`
`A cell with a membrane across which diffusible electrolyte
`distribution is purely passive would be expected to have a
`high internal concentration of sodium, suchasis true for the
`erythrocytes of some species. However, the interior of mast.
`cells is high in potassium and jow in sodium, as depicted in
`Fig 85-8. This unequal distribution of cations attests to
`special clectrolyte-transport. processes and to differential
`permeabilities of diffusible ions, so that the membrane po-
`tential is higher than that which would result from a purely
`passive Donnandistribution.
`In nerve tissue or skeletal and
`cardiac muscle, the membrane potential ranges upwards to
`about 90 mv. The electrical gradient is on the order of
`50,000 v/em, because of the extreme thinness of the mem-
`brane. Obviously, such an intense potential gradient will
`influence strongly the transmembrane passages of charged
`drug molecules.
`
`so orderly, since its composition is quite complex. Chains of
`faily acids of different degrees of saturation andcholesterol
`cannot array themselves in simple parallel arrangements.
`Furthermore, the polar heads will assume a number of orien-
`tations depending upon the substances and groups involved.
`Moreover, the lamellarportionis penetrated by large globu-
`lar proteins, the interior of which, like the lipid layers, has a
`high hydrophobicity, and some fibrous proteins.
`The plasma membrane appears to he asymmetrical, The
`lipid composition varies from cell type to cell type and per-
`haps fromsite to site on the same membrane.
`‘There are, for
`example, differences between the membrane ofthe endo-
`plasmic reticulum and the plasma membrane, even though
`the membranes are coextensive. Where membranes are
`double, the inner and outer layers may differ considerably;
`the inner and outer membranes of mitochondria have been
`shown to have strikingly different. compositions and proper-
`ties, Some authorilies have expressed doubt as to the exis-
`tence of the protein layers in biological membranes, al-
`thoughthe evidence is preponderantly in favorof at least an
`Transport is the movementof a drug from one place to
`outer glycoprotein coat. Sugar moieties also are attachedto
`another within the body. The drug may diffuse freely in
`the outer proteins; these sugar moieties are important to
`uneombined form with a kinetic energy appropriate to its
`cellular and immunological recognition and adhesion and
`thermal environment, or it may move in combination with
`have other functions as well.
`extracellular or cellular constituents, sometimes in connec-
`The cell membrane appears to be perforated by water-
`tion with energy-yielding processes that allow the molecule
`filled pores of various sizes, varying from about 4 to 10 A, the
`or complex to avercome harriers to simple diffusion.
`majority of which are about 7 A. Probably all major ion
`Simpie Nonionic Diffusion and Passive Transport—
`channels are through the large globular proteins thattra-
`Molecules in solution move in a purely random fashion,
`verse the membrane.
`‘Through these pores pass inorganic
`provided they are not charged and moving in anelectrical
`ions and small organic molecules, Since sodium ions are
`gradient. Such random movementis called diffusion;if the
`more hydrated than potassium and chloride ions, they are
`molecule is uncharged, it is called nonionte diffusion.
`larger and do notpass as freely through the pores as potassi-
`In a population of drug molecules, the probability that
`um and chloride. The vascular endothelium appears to
`during unit time any drug molecule will move across a
`have pores at least as large as 40 A, but. these seem to be
`boundary is directly proportional to the number of mole-
`interstitial passages rather than transmembrane pores.
`cules adjoining that boundary and, therefore, to the drug
`Lipid molecules small enough to pass throughthe pores may
`concentration. Except at dilutions so extreme that only a
`do so, but they have a higher prebahility of entering into the
`few molecules are present,
`the actual rate af mavement
`lipid layer, from where they will equilibrate chemically with
`(molecules/unit time) is directly proportional to the proha-
`the interior of the cell.
`Irom work on monolayers, some
`bility and, therefore, to the concentration, Once molecules
`researchers contend that it is not necessary ta postulate
`have passed through the boundaryto the opposite side, their
`pores to explain the permeability to water and sinall water-
`random motion may cause some to return and others to
`soluhle molecules.
`continue to move further away from the boundary. The rate
`Stratum Corneum—Although the stratum corneum is
`of return is ikewise proportional] to the concentration on the
`not a membrane in the same sense as a cell membrane,it
`opposite side of the boundary,
`It follows that, although
`offers a barrier to diffusion, which is of significance in the
`molecules are moving in both directions, there will he a net
`topical application of drugs. The stratum corneum consists
`movementfrom the region of higher to that of lower concen-
`of several
`layers of dead keratinized cutaneous epithetial
`tration, and the net transfer will be proportional to the
`cells enmeshed in a matrix of keratin fibers and bound to-
`concentration differential.
`If the boundary is a membrane,
`gether with cementing desmosomes and penetrating tonoli-
`which has both substance and dimension, the rale of move-
`brils of keratin. Varying amounts of lipids and fatty acids
`ment is also directly proportional to the permeability and
`from dying cells, sebumand sweat are contained among the
`inversely proportional tu the thickness.
`'These factors com-
`dead squamouscells.
`Immediately beneath the layer of
`bine into Fick’s Law of Diffusion,
`deadcells and above the viable epidermal epithelial cells is a
`dg _ PAG,~6)
`layer of keratohyaline granules and various water-soluble
`(5)
`dt
`:
`substances, such as alpha-amino acids, purines, monosac-
`charides and urea.
`Both the upper and lower layers of the stratum corneum
`are involved in the cutaneous harrier to penetration,
`‘The
`barrier to penetration from the surface is in the upperlayers
`for water-soluble substances and the lowerlayers for lipid-
`soluble substances, and the barrier to the outward move-
`ment of wateris in the towest layer.
`Membrane Potentials—-Across the cell membrane there
`exists an électrical potential, always negative on the inside
`and positive on the outside. Ha cell did not have special-
`membrane clectrolyte-transporl processes, ils membrane
`potential would be mainly the result of the Donnan equilib-
`rium (see Chapter 14} consequentto the semipermeabililyof
`the membrane. Such potentials generally lie between 2 and
`5 mv.
`
` PFIZER, INC. v. NOVO NORDISK A/S - IPR2020-01252, Ex. 1013, p. 154 of 408
`
`

`

`
`
`dGa = PIC, - Cy)
`
`(6)
`
`As diffusion continues, C) approaches C., and the net. rate,
`dQ/dl, approaches zere in exponential fashion characteristic
`of a first-order process. Equilibrium is defined as thatstate
`in which C, = Cy. The equilibriumis, of course, dynamic,
`with equal numbers of molecules being transportedin each
`direction during unit Lime.
`If wateris also moving through
`the membrane, it may eitherfacilitate (he movementof drug
`or impede il, according to the relative directions of mave-
`ment of water and drug; this effect of water movement. is
`called solvent drag.
`Yonic or Electrochemical Diffusion—-[fa drug is jon-
`ized, the transport properties are modified. The probability
`of penetrating the membrane isstill a function of concentra-
`tion, but it is also a function of the potential difference or
`electrical gradient across the membrane. A cationic drug
`molecule will be repelled from the positive charge on the
`outside of the membrane, and only those molecules with a
`high kinetic energy will pass throughthe ion barrier.
`If the
`cation is polyvalent, it may not penetrate at all,
`Once inside the membrane, a cation simultaneously will
`be attracted to the negative charge on the intracellular sur-
`face of the membrane andrepelled by the outersurface; it is
`said to be moving along the electrical gradient. Hit alsois
`moving from a higher towards a lower concentration, it is
`said to be moving alongits electrechemical gradient, which
`ig the sumof the influences of the electrical field and the
`concentration differential across the membrane.
`Onceinside the cell, cations will tend to be kept inside by
`the attractive negative charge onthe interior of the cell, and
`the intracellular concentrationof drug will increase until, by
`sheer numbers of accumulated drug particles, the outward
`diffusion or mass escape rate equals the inward transpart.
`rate, and electrochemical equilibrium is said to have ac-
`curred, At electrochemical equilibrium at body tempera-
`ture (37°C), ionized drug molecules will be distributed ac-
`cording to the Nernst equation,
`6
`C, Be
`(7)
`log C = 61
`where C, is the molar extracellular and C; the intracellular
`concentration, % is the number of charges per molecule and
`E is the membrane potential in millivolts. Log C,/C; is
`positive when the molecule is negatively charged and nega-
`tive when the moleculeis positively charged.
`Facilitated Diffusion—Sometimes a substance moves
`more rapidly through a biological membrane than can be
`accountedfor by the process of simple diffusion.
`‘This accel-
`erated movement
`is termed facilitated diffusion.
`It is
`thought to be due to the presence of a special molecule
`within the membrane, called a carrier, with which the trans-
`ported substance combines. There is considered to be a
`greater permeabilityto the carrier—drug complex than to the
`drug alone, so that the transport rate is enhanced. Afterthe
`complex traverses the membrane, it dissociates, The carrier
`musteither returnto the originalside of the membrane to be
`reused or constantly be produced on one side and eliminated
`on the otherin orderfor the carrier process to be continuous.
`Manycharacteristics of facilitated diffusion, formerly at-
`tributed Lo ion carriers, can be explained by ion exchange.
`Although facilitated diffusion resembles active transport,
`below, in its dependence upon a continuous source of energy,
`it. differs in that facilitated diffusion will only transporta
`molecule along its clect