Journal of
`Pharmaceutical
`Sciences
`
`FEBRUARY 1975
`VOLUME 64 NUMBER 2
`
`REVIEW ARTICLE
`
`Rationale for Design of Biologically Reversible
`Drug Derivatives: Prodrugs
`
`A. A. SINKULAX and S. H. YALKOWSKY
`
`Keyphrases 0 Drug derivatives, biologically reversible-rationale
`for design of prodrugs, physicochemical and biological consider-
`ations, applications, review 0 Prodrugs-rationale
`for design,
`physicochemical and biological considerations, applications, modi-
`fications affecting absorption, site direction, depot, taste and odor,
`and irritation, review 0 Reversible drug derivatives-rationale
`for
`design for prodrugs, physicochemical and biological consider-
`ations, applications, review 0 Chemical modification-prodrugs,
`physicochemical and biological considerations, applications, re-
`view Latentiated drugs-design of prodrugs, review
`
`CONTENTS
`GeneralConsiderations .................................
`183
`Physicocbemical ...................................... 183
`Absorption ........................................
`183
`Solubility ...............
`Biological .................
`Enzymes ...............
`Esterases ...............
`Alkaline Phosphatase ....
`Acid Phosphatase ........
`Sulfatases ..............
`Applications ..................................
`Site Direction .
`.................
`.............. 198
`Organoleptic Properties. .........
`.............. 200
`Taste ..................
`Mechanism of Taste Receptor Stimulation ....
`Odor .....................................
`Tastestudies ...................................... 202
`Irritation ............................................
`203
`
`Intramuscular or Intravenous Injection ............... ,203
`GIDisturbances ....................................
`204
`References .............................................
`205
`Nearly all therapeutic agents possess various phys-
`icochemical and biological properties, some desirable
`and others undesirable. In general, the pharmaceuti-
`cal world is concerned with minimizing the number
`and magnitude of undesirable properties of a drug
`while retaining the desirable therapeutic activity.
`Improvement of drug efficacy can be accomplished
`by biological, physical, or chemical means. The bio-
`logical approach entails varying the route of adminis-
`tration. Examples include the injectable route to op-
`timize onset of action, maximize bioavailability (en-
`hanced blood levels), and eliminate gastric irritation
`and acid-catalyzed drug degradation. Versatility is
`severely limited when utilizing the biological ap-
`proach, because alternative routes of administration
`are frequently unavailable and are always less conve-
`nient than oral administration.
`A greater degree of flexibility of drug modification
`is offered by the physical approach, commonly re-
`ferred to as dosage form design. The elements and
`philosophy of
`this approach were discussed by
`Schroeter (1) and others (2-8). The highest degree of
`flexibility in altering drug efficacy, however, is of-
`fered by the chemical approach.
`Drug derivatization has been long recognized as an
`important means of producing better pharmaceuti-
`
`Vol. 64, No. 2, February I975 1 181
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`cals. Bayer, as far back as 1899, synthesized the drug
`aspirin in an attempt to improve the therapeutic ac-
`tivity of salicylic acid. Since that time, literally thou-
`sands of drug derivatives have been synthesized and
`tested. These drug derivatives can be broadly classi-
`fied into two categories: irreversible or reversible. Ir-
`reversible derivatives or analogs are usually synthe-
`sized for the purpose of finding a similar, new, biolog-
`ically active entity possessing increased potency, a
`broader spectrum of activity, or some other desirable
`property not possessed by the parent compound. A
`reversible drug derivative utilizes a chemical moiety
`of proven biological activity (the parent molecule)
`and seeks to deliver it to the site of action while over-
`coming some inherent drawback to the use of the
`parent compound.
`In the case of the analog, precautions must be
`taken as to what functional group can be modifed
`since indiscriminate modification may‘ destroy all
`bioactivity. The reversible derivative can be modified
`at any functionality without undue concern for its in-
`volvement at the receptor level since it is reversible
`by definition. These facts eliminate the need to de-
`termine the bioactive center(s) in the molecule and
`offer the chemist a greater number of chemical sites
`at which to modify.
`In general, three approaches are followed in the
`search for new drug agents: (a) the general screening
`approach in which chemical substances from any
`source are tested for their effect against a predeter-
`mined disease or disease state, (b) the chemical mod-
`ification of existing drug substances whose biological
`effects are known, and (c) mimicking nature by bio-
`chemical design, where a compound is made to exert
`an action in a manner similar to a known biochemical
`substance (9). Any lead compounds obtained from
`these three approaches are usually further modified
`chemically to gain the biologically most potent rep-
`resentatives of the series.
`This review will focus on approach (b), the chemi-
`cal modification of existing drug substances whose
`biological effects are known. It will be limited solely
`to biologically reversible derivatives, i e . , those com-
`pounds that, upon introduction to the appropriate
`biological system, revert back to the parent molecule
`by virtue of enzymatic and/or chemical lability. To
`provide an interpretive review of the area, references
`from the journal and patent literature will be selected
`to illustrate the various principles discussed. The dis-
`cussion will of necessity not be comprehensive but
`will nonetheless cover the significant aspects of the
`discipline.
`Reversible derivatives (10-14) have also been
`termed prodrugs (15-22) and latentiated drugs (23-
`25) and have been designed to eliminate a variety of
`undesirable properties such as bitterness, odor, gas-
`tric upset, and poor absorption. Many comprehensive
`reviews of both reversible and irreversible drug deriv-
`atives (23-32) have appeared recently.
`Harper (25) must be credited with coining the term
`“drug latentiation,” and he defined it as “the chemi-
`cal modification of a biologically active compound to
`form a new compound, which upon in viuo enzymatic
`
`182 / Journal of Pharmaceutical Sciences
`
`:attack will liberate the parent compound.” Kupchan
`et al. (33) extended this definition operationally by
`including nonenzymatic processes as well for regener-
`ation of the parent compound. By inference, latentia-
`tion implies a time lag element or time component
`involved in regenerating the bioactive parent mole-
`cule in vivo. Since most latentiated drug substances
`per se are biologically inactive, this concept is impor-
`tant for those drugs that are metabolized or excreted
`too rapidly to provide adequate clinical efficacy.
`Albert (34, 35), in discussing the selective toxicity
`of drug molecules, elucidated the proagent or pro-
`drug concept. The term prodrug is general in that it
`includes latentiated drug derivatives as well as sub-
`stances that are converted after administration to the
`actual substance that combines with redeptors. These
`actual substances may be active metabolites of the
`parent molecule. Harper (24), on the other hand, dis-
`cussed the concept of structural formulation and de-
`fined it as “the modification of a biologically active
`compound a t a point not essential for binding to an
`active site in the biological receptor, so that although
`the desired biological effect is retained, the resulting
`changes in physicochemical properties cause alter-
`ation in the absorption, distribution or metabolism of
`the drug; the parent compound, however, is not liber-
`ated in the body.’’ Structural formulation differs
`from irreversible derivative formation in that the for-
`mer retains bioactivity in viuo whereas the latter
`may or may not do so. The distinction is subtle but
`may have profound ramifications in the rational ap-
`proach to the synthesis of bioactive agents.
`Any number of inherent disadvantages may pre-
`clude the use of the parent drug molecule in clinical
`practice. Among those properties considered disad-
`vantageous in a drug molecule are bitterness or tart-
`ness, offensive odor, gastric or intestinal upset and ir-
`ritation, pain on injection, lack of absorption, slow or
`rapid metabolism, and lack of stability in the bulk
`state, the dosage form, or in vivo (i.e., gastric insta-
`bility).
`In many cases, undesirable properties in a drug
`molecule cannot be overcome by conventional phar-
`maceutical formulation or route of administration
`changes, so the method of choice becomes reversible
`derivative formation. In the intelligent design of re-
`versible drug derivatives, it is necessary to consider
`two questions:
`1. What structural modification(s) of the parent
`molecule are necessary to reduce or eliminate the
`particular undesirable effect?
`2. What conditions are available in viuo (enzymes,
`pH, etc. ) to regenerate the parent molecule from the
`derivative?
`The first question requires an extensive knowledge
`of structure-activity relationships as they apply to
`elimination of these undesirable properties. The sec-
`ond question is dependent on a rather sophisticated
`knowledge of biology. Complete answers to these two
`questions are obviously not yet available to the medi-
`cinal chemist. A limited body of knowledge is avail-
`able, however, and this knowledge, if used judicious-
`ly, can form a basis for the rational design of revers-
`
`Patent Owner, UCB Pharma GmbH – Exhibit 2014 - 0002
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`

`
`ible drug derivatives. The remainder of this review
`will consider specific undesirable drug properties and
`possible means of eliminating these properties by
`using reversible drug derivatives. Examples of drugs
`whose properties have been successfully modified are
`provided in the tables.
`
`GENERAL CONSIDERATIONS
`Physicochemical-Absorption-Probably
`the
`most fruitful area of reversible derivatization is the
`improvement of passive drug absorption through epi-
`thelial tissue. Many studies involving in uiuo, in situ,
`and in uitro systems have been conducted to eluci-
`date the role of chemical structure in drug absorp-
`tion. The similarities and differences among some
`more widely accepted theories also will be discussed
`here.
`Nearly all of the empiricisms and theories in cur-
`rent use agree that the addition of a hydrophobic
`group to a compound usually increases its absorption.
`It is also agreed that this increase in absorption is a
`direct consequence of the increase in the biological
`lipid-water partition coefficient resulting from the
`added hydrophobic moiety. Although there is a lack
`of agreement as to what, if any, in uitro partitioning
`system best mimics the biological situation, octanol-
`water apparently is the most useful for correlative
`purposes.
`From both the physicochemical and the biological
`point of view, octanol-water is the most extensively
`studied system. Leo et al. (36) tabulated and critical-
`ly evaluated thousands of octanol-water partition
`coefficients and developed a system of rules for esti-
`mating values for compounds that have not yet been
`studied.
`It is important to realize that the theoretical and
`empirical relationships to be discussed are equally
`applicable to reversible derivatives as well as nonre-
`versible derivatives (analogs), because all of these re-
`lationships rely upon partitioning and because the
`presence of a reversible linkage does not normally
`alter the relative partition coefficients of the mem-
`bers of a series. For example, in the octanol-water
`system, the ratio of the partition coefficients of ethyl
`and butyl benzoates is the same as the ratio for p -
`ethyl- and p - butylbenzoic acids, even though the es-
`ters are reversible while the acids are not. Another
`consequence of the dependence of absorbability on
`partitioning is the fact that the unionized form of a
`dissociable molecule is absorbed more efficiently
`than its ionic species. The quantitative relationships
`between pH, pK, and partition coefficient are well
`known (37,38) and will not be discussed here.
`To get a clear picture of the role of hydrophobicity
`in drug transport, it is necessary to cover a wide
`range of partition coefficients. The most convenient
`and economical way of accomplishing this task is to
`construct a homologous series. The partition coeffi-
`cient of the nth number of any homologous series,
`PC,, in any solvent system can be described by:
`lOg(PC,) = lOg(PC,) + *"n
`
`(Eq. 1)
`
`Table Ia-Values of TCH* for Some Common Solvents
`and for Red Blood Cell Ghosts
`
`Ether
`Ether
`Octanol
`Chloroform
`Olive oil
`Castor oil
`Red blood cell ghosts
`
`0.573
`0.612
`0.500
`0.609
`0.525
`0.545
`0.526
`
`a Adapted, with permission, from Ref. 46.
`
`where PCO is a constant dependent on both the se-
`ries and the solvent system, and + is a constant de-
`pendent only on the solvent system. According to the
`notation of Leo et al. (36), a can be an incremental
`constant for any substituent; but when dealing pri-
`marily with homologous series, a will designate T C H ~
`unless otherwise specified.
`The values of T ( T C H ~ ) for several organic solvents
`and red blood cell ghosts against water are listed in
`Table I. From the octanol-water T value of 0.5, it can
`be seen that only seven consecutive homologs are
`needed to cover a 1000-fold range of partition coeffi-
`cients in half-log increments. Spanning this broad
`range of values minimizes the effects of random error
`and normal biological variation. Homologous series
`are particularly well suited for studying transport be-
`cause the alkyl group usually does not interfere with
`the interaction of the active portion of the molecule
`and the receptor site. Therefore, it is frequently pos-
`sible to measure the relative biological activities of
`homologs and then equate these values to relative
`transport rates.
`The earliest structure-activity or structure-trans-
`port workers all recognized the parallelism between
`the logarithms of the biological response (BR) and
`the lipid-water partition coefficient (PC). Equations
`of the form:
`log(BR) = a + b log(PC)
`0%. 2 )
`were proposed (39-44). For the simple case of a ho-
`mologous series, Eqs. 1 and 2 are combined giving:
`log(BR) = hs + b log(PC,) + a
`th. 3 )
`which describes the so-called linear structure-activi-
`ty relationship.
`The value of a in Eq. 2 or 3 is a measure of the de-
`gree of similarity of the in uitro and the true in uivo
`partitioning systems. If a is equal to unity, the sys-
`tems are equivalent in their relative affinities for a
`methylene group. (It is possible for T C H ~ to be the
`same for a pair of systems and, at the same time, for
`other substituent constants such as TOH to be quite
`different. For specific examples, see Ref. 36.) When
`considering a homologous series, it is not necessary to
`use any reference in uitro partitioning system. Flynn
`and Yalkowsky (45) showed that b+
`is equal to rR,
`the a value in the biological system. Thus, Eq. 3 be-
`comes:
`log(BR) = A + T H R
`
`(Eq. 4 )
`
`Val. 64, No. 2, February 1975 1 183
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`Patent Owner, UCB Pharma GmbH – Exhibit 2014 - 0003
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`

`
`c
`
`/
`
`t t
`
`10-2 I
`
`7
`6
`5
`4
`3
`2
`1
`ALKYL CHAIN LENGTH
`Figure 1-Data of Scheuplein and Blank (48) for permeation
`of normal aliphatic alcohols across human stratum corneum
`in vitro. Key: 0, concentration-normalized flux; and 0, flux
`from 0.10 M solution (last four points represent saturated
`solutions). (Adapted, with permission, from Ref. 46.)
`
`8
`
`9
`
`
`
`where A is equal to the constant portion of Eq. 3 and
`is, therefore, dependent upon both the biological sys-
`tem and the homologous series.
`Yalkowsky and Flynn (46) recently evaluated a
`large number of linear chain-length-activity relation-
`ships which are dependent upon passive transport.
`They observed, as expected, that rB for a particular
`biological system is essentially independent of the se-
`ries under study. They found that the value of 178 for
`most simple organisms (bacteria, fungus, erythro-
`cytes, etc. ) is 0.46 f 0.03; i.e., on the average, there is
`a 2.9-fold increase in activity for each additional
`methylene group. It was also observed that the value
`of xB for epithelial and GI tissue of higher animals is
`around 0.25, which corresponds to an incremental
`constant for each methylene group of about 1.8 for
`the change in activity (or transport) with chain
`length. This low value is in agreement with the fact
`that these tissues contain a significant fraction of
`polar components.
`The linear increase in the logarithm of the trans-
`port rate with chain length is expected from basic
`permeability theory. The flux, F, or transport rate of
`a substrate across a rate-determining lipid phase or
`membrane separating two aqueous phases a t a con-
`centration differential of AC is:
`
`PC
`F = -AC
`R
`where PC is the lipid-water partition coefficient of
`the substrate, and R is the resistance of the mem-
`brane to its diffusion. Collander (47), using chara
`cells, was among the first to show the relationship be-
`tween permeability and partitioning in a biological
`
`184 1 Journal of Pharmaceutical Sciences
`
`system. More recently, excellent correlation was
`shown (48) between the permeability of nine aliphat-
`ic alcohols across excised human stratum corneum
`and their stratum corneum-water partition coeffi-
`cients. From these data (Fig. l), it can be seen that
`there is a great increase in permeability between
`methanol and nonanol.
`The exponential increase in transport rate and
`thus activity with extention of the alkyl group cannot
`go on indefinitely. If experiments are carried to high
`enough chain lengths, there is a leveling off or pla-
`teauing of the curve and ultimately a decrease in ac-
`tivity with increasing hydrophobicity. While there is
`reasonably good agreement in the literature about
`the ascending portion of the structure-activity curve,
`there is a great deal of controversy about the reason
`for and even the shape of the apical or descending
`portions of the curve.
`Hansch (49) described the overall shape of the
`structure-activity curve by a parabolic equation of
`the form:
`log(BR) = a + b log(PC) + c log2(PC)
`(Eq. 6)
`He showed that Eq. 6 gives better statistical fit to
`many sets of data than the linear Eq. 2. The im-
`proved correlation is undoubtedly at least partially
`due to the greater degree of flexibility produced by
`the additional variable c; but since c is always nega-
`tive, it cannot be regarded as simply another adjust-
`able parameter. One difficulty that arises with the
`parabolic equation is that the values of the coeffi-
`cients, a and b in Eqs. 2 and 6, have no relationship
`to one another and parabolic equations having the
`same ascending slopes can appear quite different.
`Since Hansch’s parabolic relationship is based
`upon a countercurrent distribution-type model (49),
`the curve is nowhere linear and is symmetrical about
`an optimum value of log (PC). The nonlinearity of
`the parabola makes it difficult to reconcile with the
`linear case described by Eq. 2. Furthermore, the sym-
`metry is not consistent with the many reasons given
`by Hansch and Clayton (50) to explain the decrease
`in activity with chain length. Nevertheless, in spite of
`the theoretical shortcomings, the Hansch parabolic
`relationship can be extremely useful in the empirical
`analysis of structure-activity data and in the predic-
`tion of optimum partition coefficients for biological
`activity. From a practical standpoint, it is frequently
`more useful than the more theoretically valid rela-
`tionships that will be discussed.
`Wagner and Sedman (51) recently analyzed much
`of Hansch’s data and found that a statistically better
`fit is obtained with an equation of the form:
`
`Others also showed good fit of biological activity data
`and transport data to equations similar to Eq. 7 (46,
`52, 53). One of the earliest uses of this equation was
`by Zwolinski et al. (54) who studied the permeability
`of various plant cells to the members of several ho-
`mologous series. They also showed that double-recip-
`
`Patent Owner, UCB Pharma GmbH – Exhibit 2014 - 0004
`
`

`
`rocal plots [(BR)-’ uersus (PC)-’] are linear and can
`be used to evaluate the constants a and b convenient-
`ly. Another important feature of Eq. 7 is that it is ap-
`plicable to both linear and nonlinear data.
`The mathematical form of Eq. 7 can be obtained
`directly from Eq. 5 by the incorporation of an addi-
`tional resistance in series with that of the membrane.
`Equation 5 then becomes:
`
`(Eq. 8 )
`
`This added resistance, Raq, results from the unstirred
`aqueous layers adjacent to the membrane which must
`be traversed by any solute passing through the mem-
`brane. For a more complete description of the deriva-
`tion of Eq. 8 and the importance of unstirred layers
`(or diffusion layers as they are often called), the read-
`er is referred to Refs. 45 and 46. For an alternative
`treatment based on extraction theory leading to an-
`other equation of the same form as Eq. 7, see Ref. 51.
`For the study of a homologous series, it is conve-
`nient to combine Eq. 1 with the logarithmic form of
`Eq. 8 (or 7) to get:
`log(BR) = log(AC) - lOg(PC,) - ~n - log(R, + R,,PC,IO””)
`
`(Eq. 9)
`which is shown schematically in Fig. 2. It can be seen
`that, according to Eq. 9, there is a linear increase in
`log BR with chain length but that at some point
`[when R,, 2 (R,/PC)]
`the curve levels off and ap-
`proaches a limiting value. These equations can satis-
`factorily describe most structure-activity data, but
`they do not explain the descending portion of the
`curve.
`Because of the wide variety of reasons for a decline
`in activity with increasing chain length, no single
`equation can explain all available data. Hansch and
`Clayton (50) and Yalkowsky and Flynn (46) listed
`about a dozen possible reasons for this decline. These
`reasons can be broadly classified into those that are
`dependent upon a biological parameter (e.g., enzyme
`specificity, conformational distortion of the active
`site, metabolism, and poisoning of enzymes) and
`those that are related to some physical property (e.g.,
`solubility, complex formation, micelle formation,
`partitioning into inert phases, and binding to inert
`surfaces). The former are the most difficult to corre-
`late by simple theories but can be handled quite sat-
`isfactorily by equations such as Eq. 6. The latter all
`have one important feature in common; they can gen-
`erally be described mathematically by:
`lOg(P,) = log(P,,) + an
`(Eq. 10)
`where P,, is the value of any property of the n th ho-
`molog of the series, PO is a reference value, and a is a
`well-defined constant’.
`
`Occasionally, it is necessary to use a higher order polynomial of n to de-
`scribe P,,. This would alter the subsequent mathematical treatment slightly
`but not the general conclusions (see Ref. 52).
`
`>
`t
`-
`>
`I-
`0 6
`J
`6
`0
`s
`c1)
`0
`
`\
`\
`\
`\
`\
`\
`\
`\
`\
`\
`\
`
`A
`
`
`
`U
`W
`
`Lz
`0
`X 3
`k
`+ ?
`n
`l5
`
`I-
`v)
`U 0
`z
`I t
`a
`6
`0 s
`
`ALKYL CHAIN LENGTH
`Figure 2-Hypothetical chain-length activity relationships.
`(Adapted, with permission, from Ref. 52.)
`
`(Eq. 12)
`
`Yalkowsky et al. (46) theoretically characterized
`the effects of behavior on the transport rate by using
`solubility as an example. Based on literature data for
`nearly 20 homologous series in water, they found’:
`log(S,) = log(&) - dn
`(Eq. 11)
`where Sn is the solubility of the n th member of the
`series, and S O is a constant.
`The effect of solubility on transport rate is to limit
`the attainable concentration differential so that the
`maximum value of Eq. 8 becomes:
`K,)
`S
`- -
`( R J P C ) + R,,
`PColOrn + R,,
`F =
`which, in logarithmic form, is:
`log(F) = log(S,PCJ + ( H - 6)n - log(R,, + R,,,PCJO”“)
`(Eq. 13)
`These equations can now describe a “parabolic”
`structure-activity curve on the basis of transport-
`limited activity and basic physical-chemical relation-
`ships. Figure 2 shows the expected dependence of
`transport across a biological barrier (and activity de-
`pendent thereupon) for the members of a homolo-
`gous series predicted by Eqs. 9 and 13. The scales are
`arbitrary but show that the break occurs at the same
`chain length for the equimolar and saturated cases.
`Figure 3 shows an experimental verification of Eqs. 9
`and 13. These data were obtained from turnover time
`experiments with goldfish (52). The agreement be-
`tween experimental and theoretical data, while not
`necessarily proving the theory, gives a positive indi-
`cation of its utility.
`Solubility -The primary role of solubility in de-
`termining drug absorption is obvious since only the
`
`Vol. 64, No. 2, February 1975 1185
`
`Patent Owner, UCB Pharma GmbH – Exhibit 2014 - 0005
`
`

`
`,' .
`
`_ _ - - -
`
`I
`
`C ._ E
`
`a
`W >
`0 z
`a
`3
`t-
`J d 0" 10-1
`a a s
`
`CL
`
`10-2 I
`0
`
`'I
`
`1
`
`6
`5
`4
`3
`2
`ESTER CHAIN LENGTH, n
`Figure 3-Turnover
`times produced by various concentrations
`and saturated solutions of n-alkyl p-aminobenzoates. Points
`are experimental: 0, unsaturated; and 0 , saturated. Lines are
`theoretical: --, unsaturated; and -,
`saturated. (Reprinted,
`with permission, from Ref. 52.)
`
`7
`
`
`
`drug that is in solution is available for absorption.
`Since virtually any structural modification that alters
`solubility will also alter ionization and partitioning, it
`is somewhat difficult to provide clearcut examples of
`the relationship between solubility and absorption.
`Some effects of structure on aqueous solubility will
`be discussed later. The importance of solubility was
`apparent in the previous discussion of homologous
`series. It frequently is the factor responsible for the
`parabolic shape of many structure-activity curves.
`For homologous series, solubility in aqueous media
`generally decreases by a factor of 4.0 for each meth-
`ylene unit. Branched alkyl moieties decrease aqueous
`solubility to a lesser extent than linear chains. It was
`shown that this results because the former have a
`lower surface area than the latter and because the
`solubility of over 70 aliphatic alcohols (linear, cyclic,
`and branched) is directly proportional to the hydro-
`carbon surface area (55). An earlier study (56)
`showed a similar correlation for hydrocarbons. Many
`empirical correlations have appeared which are use-
`ful for estimating the aqueous solubility of organic
`liquids (57-59). These correlations, while far from
`complete, can be of value in evaluating the effect of a
`structural modification on solubility.
`Since many drugs are either weak acids or weak
`bases or their salts, dissociation must be regarded as
`an important factor in determining absorbability. It
`is generally known (37,38) that the unionized form of
`a drug is absorbed far more efficiently than the ionic
`species, even though the latter is more soluble. The
`explanation for this observation lies in the fact that
`the increase in the partition coefficient in going from
`a salt to free acid (or free base) usually exceeds the
`corresponding decrease in solubility by several orders
`
`186 / Journul of Pharmaceutical Sciences
`
`of magnitude. This is analogous to T being greater
`than 6 in Eq. 13. The greater absorbability of the un-
`ionized species over the salt is further amplified by
`the fact that it is rarely practical to give saturated so-
`lutions of the salt.
`Many workers (37, 38) regard the absorption of
`ionic species as nonexistent and treat the undisso-
`ciated species in a manner similar to the treatment of
`nonelectrolytes discussed. Others (60,61) have shown
`that certain ionic drugs are absorbed in their undis-
`sociated state, either directly or by ion-pair or com-
`plex formation. While these mechanisms are certain-
`ly operative in specific instances, they are not of suf-
`ficient importance to be of concern here. Conse-
`quently, the term AC in Eqs. 5, 8, and 9 must refer
`only to the concentration differential of the undisso-
`ciated form of the drug. In each aqueous phase, the
`concentration of unionized species, C,, can be related
`to the total concentration, Ct, the pH, and the pK of
`the weak acidic drug by:
`
`(Eq. 14)
`
`A similar equation can be written for basic drugs.
`The combination of Eqs. 14 and 5 is known as pH-
`partition theory. The direct application of this theory
`to gastric and intestinal absorption has been only
`partially successful. The data seem to indicate that
`the intestinal pH is closer to 5.5 than it is to the ac-
`cepted value of 7.4. This observation led to the postu-
`lation of a region of the intestinal lumen adjacent to
`its surface which has a pH of 5.5 and which is in equi-
`librium with the bulk of the lumen (37). This virtual
`pH hypothesis has been criticized (62) because its ex-
`istence would have no effect on the amount of union-
`ized drug at the luminal surface.
`Dissociation behavior, however, is important in de-
`signing reversible derivatives when the linkage in-
`volves the ionizable group. Thus, a pH-sensitive drug
`such as 15-methylprostaglandin Faa becomes unioni-
`zable when converted to its methyl ester.
`If absorption is to be increased by adjusting hydro-
`phobicity, a linkage must be selected that will remain
`intact until absorption is complete, with subsequent
`release of the parent molecule into the bloodstream
`or at some specified tissue. To choose such a linkage
`rationally, an awareness of what linkage-cleaving en-
`zymes are present in the GI tract, the liver, the blood,
`and the various body tissues is necessary.
`Biological-Enzyrnes-In
`a broad sense, the basis
`for the rational design of biologically reversible drug
`derivatives is predicated on the ability of the host tis-
`sue or organism to regenerate the drug derivative to
`the bioactive parent molecular species. The manner
`in which this is frequently accomplished is through
`the mediation of an enzyme or enzyme system within
`the host. In this respect, Bender (63) and Jencks (64)
`reviewed the mechanisms of enzyme catalysis. The
`enzyme(s) may be widely distributed throughout the
`host tissue (e.g., esterase) or localized in site-specific
`tissue (e.g., amidase). Enzymes per se have been
`characterized chemically, but their efficiency and
`
`Patent Owner, UCB Pharma GmbH – Exhibit 2014 - 0006
`
`

`
`Table 11-Reversible Derivative and Prodrug Linkages and Enzymes Responsible for Their In Vim Hydrolysis
`
`Linkage
`
`Hydrolyzing Enzyme
`
`Tissue
`
`Reference
`
`Ester
`Short-medium chain
`aliphatic
`
`Long chain aliphatic
`Carbonate
`
`Hemiester
`Phosphate, organic
`
`Pyrophosphate
`Sulfate, organic
`
`Amide
`
`Amino acid
`
`Azo
`
`C holinesterase
`Ester hydrolase
`Lipase
`Cholesterol esterase
`Acetylcholinesterase
`Acetyl esterase
`Aldehyde oxidase
`Lipase
`Carboxypeptidase
`Pancreatic lipase
`Pancreatin
`Lipase
`Aliesterase
`Carboxypeptidase
`Cholinesterase
`Esterases
`Acid phos hatase
`Alkaline pgosphatase
`Acid phosphatase I11
`Pyrophosphatase I
`Steroid sulfatase
`Arylsulfohydrolase A and B
`(phenolsulfatase, arylsul-
`fatase)
`Arylsulfohydrolase C
`Estrogen sulfohydrolase
`Steroid-3~-sulfohydrolase
`Steroid-21-sulfat ase
`Amidase
`
`Proteolytic enzymes
`Chymotrypsins A and B
`Trypsin
`Carboxypeptidase A
`Carboxypeptidase B
`Azoreductase
`
`Carbamate
`
`Carbamidase
`
`Phosphamide
`Glucosiduronate
`(6-glucuronide)
`
`Phosphoramidases
`8-Glucuronidase
`
`N- Acet ylglucosaminide
`8-Glucoside
`
`a-N-Acetylglucosaminidase
`p-N-Acetylglucosaminidase
`p-Glucosidase
`
`Liver, kidney, gut
`Blood, intestinal mucosa
`
`149-160, 161,
`162
`
`General distribution
`Liver
`Gut
`Intestine
`
`Blood
`Blood
`Blood
`Blood
`Blood
`Liver, gut, blood
`Liver, blood
`Liver
`
`Liver
`Liver, gut
`Liver
`Placenta
`Liver
`Neoplastic tissue
`Walker carcinosarcoma 256
`Dunning rat leukemia
`Neoplastic tissue
`Gut
`Neoplastic tissue
`
`Liver
`Walker rat carcinoma
`Sarcoma 180
`Adenocarcinoma 755
`Lymphoid leukemia L-1210
`Liver
`Gut microflora
`Liver
`Walker carcinosarcoma
`256 (rats)
`Adenocarcinoma 755 (mice)
`Liver
`Neoplastic tissue (liver)
`Liver
`Liver
`Gut
`Blood
`Gut microflora
`Liver, gut, blood
`Gut, liver
`Gut microflora
`
`163
`
`105, 107
`103, 116
`164
`165-168
`
`169
`170-173
`174, 175
`176
`177
`178, 179
`
`180
`~~ 181, 182
`183-185
`186
`156, 187-193
`194
`195
`191
`105, 107, 196
`110
`106
`io5, 107
`105, 107
`108, 109, 197,
`198
`199
`200