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`Chem. Rev. 1996, 96, 3147- 3176
`
`Bioisosterism: A Rational Approach in Drug Design
`
`George A. Patani and Edmond J. LaVoie*
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`3147
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`Department of Pharmaceutical Chemistry, College of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08855-0789
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`Received May 15, 1996(Revised Manuscript Received July 25, 1996)
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`Contents
`I.
`Introduction
`II. Classical Bioisosteres
`A. Monovalent Atoms or Groups
`1. Fluorine vs Hydrogen Replacements
`2.
`Interchange of Hydroxyl and Amino
`Groups
`Interchange of Hydroxyl and Thiol Groups 3151
`3.
`4. Fluorine and Hydroxyl, Amino, or Methyl
`3152
`Groups as Replacements for Hydrogen
`(Grimm’s Hydride Displacement Law)
`5. Monovalent Substitutions Involving
`Chloro, Bromo, Thiol, and Hydroxyl
`Groups (Erlenmeyer’s Broadened
`Classification of Grimm’s Displacement
`Law)
`B. Divalent Isosteres
`1. Divalent Replacements Involving Double
`Bonds
`2. Divalent Replacements Involving Two
`Single Bonds
`C. Trivalent Atoms or Groups
`D. Tetrasubstituted Atoms
`E. Ring Equivalents
`1. Divalent Ring Equivalents
`2. Trivalent Ring Equivalents
`III. Nonclassical Bioisosteres
`A. Cyclic vs Noncyclic Nonclassical Bioisosteric
`Replacements
`B. Nonclassical Bioisosteric Replacements of
`Functional Groups
`1. Hydroxyl Group Bioisosteres
`2. Carbonyl Group Bioisosteres
`3. Carboxylate Group Bioisosteres
`4. Amide Group Bioisosteres
`5. Thiourea Bioisosteres
`6. Halogen Bioisosteres
`IV. Conclusion
`V. Acknowledgments
`VI. References
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`George Patani graduated with a B.Pharm. in 1992 from the College of
`Pharmaceutical Sciences, Mangalore University at Manipal, India.
`In 1996,
`he received his M.S. in Pharmaceutical Science at Rutgers University
`under the direction of Professor Edmond J. LaVoie. He is presently
`pursuing graduate studies in pharmaceutics. His current research interests
`are focused on drug design and controlled drug delivery.
`
`Edmond J. LaVoie received his B.S. in Chemistry from Fordham University
`in 1971 and his Ph.D. in Medicinal Chemistry from S.U.N.Y. at Buffalo
`under the direction of Dr. Wayne K. Anderson. After postdoctoral study
`with Dr. S. Morris Kupchan at the University of Virginia, he joined the
`American Health Foundation in Valhalla, NY.
`In 1988, he was appointed
`Professor of Medicinal Chemistry in the College of Pharmacy at Rutgers
`University. His current research interests are in the design and synthesis
`of cancer chemotherapeutics and in the elucidation of mechanism(s) of
`carcinogenesis.
`for the rational modification of lead compounds into
`safer and more clinically effective agents. The con-
`cept of bioisosterism is often considered to be qualita-
`tive and intuitive.1
`The prevalence of the use of bioisosteric replace-
`ments in drug design need not be emphasized. This
`topic has been reviewed in previous years.2-5 The
`objective of this review is to provide an overview of
`bioisosteres that incorporates sufficient detail to
`enable the reader to understand the concepts being
`delineated. While a few popular examples of the
`successful use of bioisosteres have been included, the
`
`I. Introduction
`Years of cumulative research can result in the
`development of a clinically useful drug, providing
`either a cure for a particular disease or symptomatic
`relief from a physiological disorder. A lead compound
`with a desired pharmacological activity may have
`associated with it undesirable side effects, charac-
`teristics that limit its bioavailability, or structural
`features which adversely influence its metabolism
`and excretion from the body. Bioisosterism repre-
`sents one approach used by the medicinal chemist
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`S0009-2665(95)00066-5 CCC: $25 00
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`© 1996 American Chemical Society
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`3148 Chemical Reviews, 1996, Vol. 96, No. 8
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`present review is focused primarily upon specific
`examples from current literature. The emphasis in
`this review was to outline bioisosteric replacements
`which have been used to advance drug development.
`No attempt was made to be exhaustive or to illustrate
`all of the specific analogues represented within a
`single study.
`The ability of a group of bioisosteres to elicit similar
`biological activity has been attributed to common
`physicochemical properties. In this review an at-
`tempt has been made to quantitate,
`in specific
`instances, physicochemical effects such as electro-
`negativity, steric size, and lipophilicity and to cor-
`relate these values to the observed biological activity.
`Thus, an additional objective of this review was to
`demonstrate the opportunities that one has in em-
`ploying bioisosteres to gain more specific insight into
`the quantitative structure-activity relationships
`(QSAR) associated with a specific class of drugs.
`While in some instances such associations were
`detailed by the authors of these literature examples,
`others were developed on the basis of evident cor-
`relations. To further explain and rationalize the
`biological activity observed with nonclassical bioiso-
`steric groups, the observed biological activity has also
`been correlated with some substituent constants
`commonly employed in QSAR studies. These obser-
`vations are consistent with the fact that bioisosteric
`replacements often provide the foundation for the
`development of QSAR in drug design.4,6 Recent
`advances in molecular biology, such as cloning of the
`various receptor subtypes, have enabled a clearer
`definition of the pharmacophoric sites. Bioisosteric
`replacements of functional groups based on this
`understanding of the pharmacophore and the phys-
`icochemical properties of the bioisosteres have en-
`hanced the potential for the successful development
`of new clinical agents.
`The bioisosteric rationale for the modification of
`lead compounds is traced back to the observation by
`Langmuir in 1919 regarding the similarities of vari-
`ous physicochemical properties of atoms, groups,
`radicals, and molecules.7 Langmuir compared the
`physical properties of various molecules such as N2
`- and NCO- and found
`and CO, N2O and CO2, and N3
`them to be similar. On the basis of these similarities
`he identified 21 groups of isosteres. Some of these
`groups are listed in Table 1. He further deduced from
`the octet theory that the number and arrangement
`of electrons in these molecules are the same. Thus,
`isosteres were initially defined as those compounds
`or groups of atoms that have the same number and
`
`Table 1. Groups of Isosteres as Identified by
`Langmuir
`groups
`1
`2
`3
`4
`V
`8
`9
`10
`V
`20
`21
`
`isosteres
`H-, He, Li+
`O2-, F-, Ne, Na+, Mg2+, Al3+
`S2-, Cl-, Ar, K+, Ca2+
`Cu2-, Zn2+
`V
`N2, CO, CN-
`+
`CH4, NH4
`-, CNO-
`CO2, N2O, N3
`V
`2-MnO4-, CrO4
`
`
`3-SeO42-, AsO4
`
`
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`Patani and LaVoie
`
`arrangement of electrons. He further defined other
`relationships in a similar manner. Argon was viewed
`as an isostere of K+ ion and methane as an isostere
`+ ion. He deduced, therefore, that K+ ions and
`of NH4
`+ ions must be similar because argon and meth-
`NH4
`ane are very similar in physical properties. The
`biological similarity of molecules such as CO2 and
`N2O was later coincidentally acknowledged as both
`compounds were capable of acting as reversible
`anesthetics to the slime mold Physarum polyceph-
`alum.8
`A further extension to this concept of isosteres
`came about in 1925 with Grimm’s Hydride Displace-
`ment Law.9,10 This law states: “Atoms anywhere up
`to four places in the periodic system before an inert
`gas change their properties by uniting with one to
`four hydrogen atoms, in such a manner that the
`resulting combinations behave like pseudoatoms,
`which are similar to elements in the groups one to
`four places respectively, to their right.” Each vertical
`column as illustrated in Table 2, according to Grimm,
`would represent a group of isosteres.
`Table 2. Grimm’s Hydride Displacement Law
`C
`N
`O
`F
`Ne
`CH
`NH
`OH
`FH
`CH2
`NH2
`OH2
`CH3
`NH3
`CH4
`
`Na
`-
`+
`FH2
`+
`OH3
`+
`NH4
`
`4
`N+
`P+
`S+
`As+
`Sb+
`
`Erlenmeyer11 further broadened Grimm’s clas-
`sification and redefined isosteres as atoms, ions, and
`molecules in which the peripheral layers of electrons
`can be considered identical (Table 3).
`Table 3. Isosteres Based on the Number of
`Peripheral Electrons
`no. of peripheral electrons
`8
`5
`6
`7
`ClH
`P
`S
`Cl
`BrH
`As
`Se
`Br
`IH
`Sb
`Te
`I
`SH2
`PH
`SH
`PH2
`PH3
`The widespread application of the concept of iso-
`sterism to modify biological activity has given rise
`to the term bioisosterism. As initially defined by
`Friedman,2 bioisosteres were to include all atoms and
`molecules which fit the broadest definition for iso-
`steres and have a similar type of biological activity,
`which may even be antagonistic. More recently this
`definition has been broadened by Burger as “Com-
`pounds or groups that possess near-equal molecular
`shapes and volumes, approximately the same distri-
`bution of electrons, and which exhibit similar physi-
`cal properties...”.5 The critical component for bio-
`isosterism is that bioisosteres affect the same phar-
`macological target as agonists or antagonists and,
`thereby, have biological properties which are related
`to each other.
`Bioisosteres have been classified as either classical
`or nonclassical.12 Grimm’s Hydride Displacement
`Law and Erlenmeyer’s definition of isosteres outline
`a series of replacements which have been referred
`to as classical bioisosteres. Classical bioisosteres
`have been traditionally divided into several distinct
`categories:
`(A) monovalent atoms or groups; (B)
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`Bioisosterism: A Rational Approach in Drug Design
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`divalent atoms or groups; (C) trivalent atoms or
`groups; (D) tetrasubstituted atoms; and (E) ring
`equivalents.
`Nonclassical isosteres do not obey the steric and
`electronic definition of classical isosteres. A second
`notable characteristic of nonclassical bioisosteres is
`that they do not have the same number of atoms as
`the substituent or moiety for which they are used as
`a replacement. Nonclassical bioisosteres can be
`further divided into groups:
`(A) rings vs noncyclic
`structures; and (B) exchangeable groups.
`This approach to classifying bioisosteres will be
`used to review literature examples of those bioiso-
`steric replacements that have provided useful infor-
`mation on the structure-activity relationships as-
`sociated with various pharmacologically active com-
`pounds.
`
`II. Classical Bioisosteres
`
`A. Monovalent Atoms or Groups
`Similarities in certain physicochemical properties
`have enabled investigators to successfully exploit
`several monovalent bioisosteres. These can be di-
`vided into the following groups:
`(1) fluorine vs
`hydrogen replacements; (2) amino-hydroxyl inter-
`changes; (3) thiol-hydroxyl interchanges; (4) fluorine,
`hydroxyl, amino, and methyl group interchanges
`(Grimm’s Hydride Displacement Law); (5) chloro,
`bromo, thiol, and hydroxyl group interchanges (Er-
`lenmeyer’s Broadened Classification of Grimm’s Dis-
`placement Law).
`
`1. Fluorine vs Hydrogen Replacements
`The substitution of hydrogen by fluorine is one of
`the more commonly employed monovalent isosteric
`replacements. Steric parameters for hydrogen and
`fluorine are similar, their van der Waal’s radii being
`1.2 and 1.35 Å, respectively.13 Thus, the difference
`in the electronic effects (fluorine being the most
`electronegative element in the periodic table) is often
`the basis for the major differences in the pharmaco-
`logical properties of agents where fluorine has been
`substituted for hydrogen. Due to its electronegativ-
`ity, fluorine exerts strong field and inductive effects
`on the adjacent carbon atom. Fluorine substitution,
`in general, exerts a diminished electron-withdrawing
`effect at distal sites. However, fluorine can donate
`a lone pair of electrons by resonance. This is com-
`monly referred to as its mesomeric effect. The
`opposing resonance and field effects can nearly
`cancel. The pharmacological differences can be at-
`tributed to the influence of the electron-withdrawing
`effect that the fluorine substitution causes on inter-
`action with either a biological receptor or enzyme,
`as well as its effect on the metabolic fate of the drug.
`The antineoplastic agent 5-fluorouracil (5-FU) rep-
`resents a classical example of how fluorine substitu-
`tion of a normal enzyme substrate can result in a
`derivative which can alter select enzymatic processes.
`In this instance, 5-FU is biochemically transformed
`in vivo into 5-fluoro-2¢
`-deoxyuridylic acid. Its close
`similarity to uracil allows this fluoro derivative to be
`a successful mimetic. This biochemically altered
`
`Chemical Reviews, 1996, Vol. 96, No. 8 3149
`form of 5-FU, 5-fluoro-2¢
`-deoxyuridylic acid, is ulti-
`mately responsible for the inhibition of thymidylate
`synthase, an enzyme involved in the conversion of
`uridylic acid to thymidylic acid and critical for DNA
`synthesis (Figure 1). The increased reactivity of
`5-fluoro-2¢
`-deoxyuridylic acid relative to 2¢
`-deoxy-
`uridylic acid is due to the inductive effect of fluorine
`which results in its covalent binding to thymidylate
`synthase.
`
`Figure 1.
`The application of the monovalent substitution of
`a fluorine atom for a hydrogen atom can also be seen
`in a more recent study with naphthyl-fused diaze-
`pines, which were employed as agonistic probes of
`the pharmacophore of benzodiazepine receptors.14
`Replacement of the hydrogen with fluorine at the
`ortho position of the pendent phenyl group of either
`naphthyl-fused diazepines, as illustrated in Figure
`2, resulted in enhanced affinity and efficacy for both
`naphthyl isomers (Table 4). This greater receptor
`binding affinity could again be attributed to the
`inductive effect of the fluorine atom facilitating a
`stronger interaction with the receptor.
`
`Figure 2.
`
`Table 4. Benzodiazepine Receptor Binding Affinity
`for Naphthyl-Fused Diazepines
`IC50 (nM)a
`compound
`X
`1a
`1000
`H
`1b
`260
`F
`2a
`1000
`H
`2b
`55
`F
`a In vitro potency of the compound to displace [3H]fluni-
`trazepam from the benzodiazepine receptor.
`
`Another good illustration of this monovalent bioi-
`sosteric replacement is observed in a recent series of
`anti-inflammatory corticosteroid analogues (3, Figure
`3).15
`In this study, the topical anti-inflammatory
`activity of two pairs of structurally similar corticos-
`teroids were compared. Their relative anti-inflam-
`matory activity was normalized to fluocinolone ace-
`tonide, which was assigned a potency of 100. Table
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`3150 Chemical Reviews, 1996, Vol. 96, No. 8
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`Patani and LaVoie
`
`Figure 3.
`
`Table 5. Biological Activities of Halomethyl
`Androstane-17(cid:226)-carbothionates
`
`topical
`anti-inflammatory
`activitya
`Z
`Y
`X
`compound
`3a
`42
`dCH2
`F
`H
`3b
`dCH2
`F
`F
`108
`3c
`(cid:226)-CH3
`H
`H
`27
`3d
`(cid:226)-CH3
`F
`H
`41
`a Topical anti-inflammatory activity was measured in mice
`by modifications of the croton oil ear assay.16 Fluocinolone
`acetonide served as a positive control and is assigned a relative
`potency index of 100.
`
`5 shows that, in the case of the pair of compounds
`possessing a 16-methylene substituent, the presence
`of an additional fluorine atom at the 6R position
`results in a derivative with greater activity than 3a
`or fluocinolone acetonide. With the pair of corticos-
`teroids with a 16-methyl substituent (Z ) CH3),
`replacement of hydrogen with fluorine at the 9R
`position, 3d, also increased anti-inflammatory activ-
`ity relative to 3c.
`Thus, the ability of fluorine to replace hydrogen is
`an effective method of exploring the affinity of an
`agent to the target site (receptor or enzyme) by virtue
`of its greater electronegativity while other param-
`eters such as steric size and lipophilicity17 are
`maintained.
`
`2. Interchange of Hydroxyl and Amino Groups
`The monovalent interchange of amino and hydroxyl
`groups is well known and has been successfully
`employed in the development of various pharmaco-
`logical agents. The similar steric size (Table 7),
`spatial arrangement, and the ability of these func-
`tional groups to act as either hydrogen bond acceptors
`or donors is likely responsible for their successful use
`as bioisosteres.
`Several medicinal agents under investigation as
`potential clinical agents carry heteroaromatic moi-
`eties. Many of these heteroaromatic compounds are
`capable of tautomerization. The prototropic tautom-
`erism of heteroaromatic compounds includes all
`agents wherein a mobile proton can move from one
`site to another within the heteroaromatic molecule.
`Figure 4 illustrates one of the more common types
`of tautomerization involving the movement of a
`proton between a cyclic nitrogen atom and a sub-
`stituent on the neighboring carbon atom within the
`ring. Tautomerism in heterocyclic molecules has
`been extensively studied.18 In the presence of electron-
`donating atoms such as nitrogen in heterocyclic
`systems, it is known that there will be substantial
`tautomerization where a neighboring CsOH will
`tautomerize to CdO.19 In the case of a neighboring
`
`Figure 4.
`carbon containing CsNH2 (7, Figure 4), the preferred
`tautomer is the CsNH2 form.
`Perhaps the best known example of classical iso-
`steric substitution of an amino for a hydroxyl group
`is illustrated by aminopterin (8b) wherein the hy-
`droxyl substituent of folic acid (8a) has been substi-
`tuted by an amino group (Figure 5). As previously
`noted, this represents a monovalent bioisosteric
`substitution at a carbon atom adjacent to a hetero-
`cyclic nitrogen atom. Thus, this bioisosteric replace-
`ment has the capability of mimicking even the
`tautomeric forms of folic acid. The similarity as well
`as the capability of the amino group to hydrogen bond
`to the enzyme are two important factors that facili-
`tate the binding of aminopterin to the enzyme dihy-
`drofolate reductase.
`
`Figure 5.
`
`Interchange of an amino group with a hydroxyl
`moiety in the case of 6,9-disubstituted purines (Table
`6) has been shown to result in the development of
`agents with similar benzodiazepine receptor binding
`activity.20 This example further substantiates the
`ability of the amino group to mimic the hydroxyl
`group at the receptor site. In this study a series of
`6,9-disubstituted purines were tested for their ability
`to bind to the benzodiazepine receptor in rat brain
`tissue. The relative activity of the 9-(3-aminophenyl)-
`methyl derivative (9a) was compared to the 9-(3-
`hydroxyphenyl)methyl analogue (9b) (Figure 6). In
`contrast to aminopterin where a dramatic difference
`in binding affinity was observed relative to the
`normal substrate, these bioisosteric 6,9-disubstituted
`
`Figure 6.
`
`Table 6. Benzodiazepine Receptor Binding Activity
`of Substituted 6-(Dimethylamino)-9-benzyl-9H-purines
`compound
`R
`IC50 ((cid:237)M)a
`9a
`NH2
`0.9
`9b
`OH
`1.2
`a Concentration of compound that decreased specific binding
`of 1.5 nM [3H]diazepam to rat brain receptors by 50%.
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`Bioisosterism: A Rational Approach in Drug Design
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`Chemical Reviews, 1996, Vol. 96, No. 8 3151
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`purines exhibited similar activity with regard to their
`affinity for the benzodiazepine receptor.
`In this
`example of bioisosteric replacement, pharmacological
`activity was retained. It is important to note that
`retention of biological activity based on in vitro data
`can be critical in those instances where differences
`between bioisosteric analogues exist with regard to
`in vivo parameters which may include absorption,
`distribution, metabolism, or elimination. While one
`may only observe retention of activity associated with
`interaction of drug with the pharmacophore, bioiso-
`steres may differ dramatically in their in vivo ef-
`ficacy. Additional examples of this bioisosteric re-
`placement will be discussed in the next section on
`monovalent replacement of hydroxyl and thiol groups.
`3. Interchange of Hydroxyl and Thiol Groups
`The interchange of thiol for hydroxyl can be con-
`sidered as an extension of the amino-hydroxyl
`replacement and has been used extensively in me-
`dicinal chemistry. This replacement is based on the
`ability of both these functional groups to be hydrogen
`bond acceptors or donors. A classical illustration of
`this replacement being guanine (10a) and 6-thiogua-
`nine (10b, Figure 7).21
`
`Figure 7.
`As discussed in the previous section, when part of
`a heteroaromatic ring, these functional groups can
`exist in different tautomeric forms. Figure 8 il-
`lustrates the most common example wherein a mobile
`proton on a nitrogen atom in the aromatic ring can
`be transferred to the heteroatom attached to the
`adjacent carbon resulting in the different tautomers.
`
`Figure 9.
`
`Table 7. Calcium Channel Blocking Activity of
`1,4-Dihydropyrimidines
`
`van der Waal’s
`IC50 (nM)a
`radius24 (Å)
`X
`compound
`15a
`140
`1.40
`dO
`15b
`160
`1.50
`dNH
`15c
`17
`1.85
`dS
`a Concentration that produced 50% inhibition and deter-
`mined for the vasorelaxant activity with potassium-depolarized
`rabbit thoracic aorta.
`
`analogues with similar potency. However, substitu-
`tion with the thiol resulted in enhanced potency
`(Table 7). This could be explained by the fact that
`the size of the substituents, described here as the van
`der Waal’s radii, and the ability to hydrogen bond
`were the important factors influencing retention of
`activity. Therefore, replacement with the amino
`group, which has a similar size, resulted in similar
`potency. However, replacement with the sterically
`optimal thiol resulted in an analogue which was an
`order of magnitude more potent.
`The use of this replacement in the design of novel
`anti-inflammatory agents substantiates its utility as
`a monovalent bioisostere. Long term use of nonste-
`roidal anti-inflammatory drugs (NSAIDs) for the
`treatment of rheumatoid arthritis and other inflam-
`matory diseases has been associated with side effects
`such as gastrointestinal ulceration, bleeding, and
`nephrotoxicity.25,26 With a view to designing new
`drugs with an improved safety profile, certain thia-
`zoles (16, Figure 10 and Table 8) that are dual
`
`Figure 8.
`In the case of 6-thioguanine, the ability of this
`bioisosteric analogue to be viewed as a substrate by
`the salvage pathway associated with purine biosyn-
`thesis, allows for its transformation into 6-thiogua-
`nylic acid by hypoxanthine-guanine phosphoribosyl-
`transferase (HGPRT). However, the significance of
`this “fraudulent” nucleic acid with respect to its
`lethality to neoplasms is uncertain.22 It is as this
`phosphoriboside that either the de novo synthesis of
`nucleic acids is inhibited or incorporation into deoxy-
`ribonucleic acid occurs.
`In an attempt to enhance the calcium channel
`blocking capacity of certain dihydropyrimidine agents,
`a number of isosteric analogues with the general
`structure 15 (Figure 9) were synthesized.23 Substitu-
`tion of the hydroxyl with an amino resulted in
`
`Figure 10.
`Table 8. Anti-inflammatory Activity of Benzylidene
`Derivatives in Intact Rat Basophilic Leukemia
`(RBL-1) Cells
`
`IC50 ((cid:237)M)a
`5-LO
`CO
`electronegativity29
`Z
`compound
`16a
`1.4
`0.35
`3.51
`OH
`16b
`NH2
`0.77
`0.39
`2.61
`16c
`0.38
`0.012
`2.32
`SH
`a Concentration of the test compound causing 50% inhibition
`of 5-LO or CO formation.
`
`inhibitors of both cyclooxygenase (CO) and 5-lipoxy-
`genase (5-LO) are being studied as potential anti-
`inflammatory agents.27 The beneficial effects of
`NSAIDs have been attributed to the inhibition of the
`enzyme cyclooxygenase, thereby preventing produc-
`tion of pro-inflammatory prostaglandins.28 Leuko-
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`trienes produced by the 5-lipoxygenase enzyme path-
`way may also contribute to both inflammation and
`NSAID-induced effects. Table 8 summarizes the
`concentrations of test compounds required to cause
`a 50% inhibition of 5-LO and CO formation.
`Replacement of the hydroxyl with an amino group
`resulted in more potent activity toward 5-LO while
`the potency toward CO remained the same. How-
`ever, replacement with a thiol resulted in enhanced
`potency toward both 5-LO and CO. Comparison of
`the electronegativity values of oxygen, nitrogen and
`sulphur (Table 8) suggests that this could be a factor
`that modulates the degree of inhibition of 5-LO. Size,
`however, may play a significant role with regard to
`inhibition of CO (Table 7). Thus, the thiol group may
`be a suitable and informative bioisostere for the
`amino and hydroxyl groups in several different series
`of medicinal agents by the virtue of its size, lower
`electronegativity, and ability to hydrogen bond.
`4. Fluorine and Hydroxyl, Amino, or Methyl Groups as
`Replacements for Hydrogen (Grimm’s Hydride
`Displacement Law)
`This monovalent group of isosteres is a result of
`the direct adaptation of Grimm’s Hydride Displace-
`ment Law. The basis for the fluorine-hydrogen
`interchange and the hydroxyl-amino interchange
`was discussed previously. The existence of this
`larger group of isosteres might be attributable to a
`greater tolerance of the different physicochemical
`parameters of these functionalities within a particu-
`lar series of agents. However, in the studies outlined
`in this section, an attempt was made to correlate a
`physicochemical parameter of this group of bioisos-
`teres with the observed effect on biological activity.
`In designing agents for the treatment of cardio-
`vascular diseases, it may be beneficial to associate
`the hypotensive effects resulting from the inhibition
`of angiotensin II formation with the diuretic and
`natriuretic responses. Diuretic and natriuretic ef-
`fects can be mediated by protection of the endogenous
`atrial natriuretic peptide (ANP) from inactivation by
`inhibition of epithelial neutral endopeptidase (NEP).
`Inhibition of angiotensin II formation may be brought
`about by inhibition of endothelial angiotensin-
`converting enzyme (ACE). A series of dual metal-
`lopeptidase inhibitors have been designed on the
`basis of the characteristics of the active sites of both
`enzymes. Monovalent substitution by fluorine, hy-
`droxyl, and amino in place of hydrogen has recently
`been used in the design of these metallopeptidase
`inhibitors (Figure 11, Table 9).30
`In this study optically pure N-[2-(mercaptomethyl)-
`3-phenylbutanoyl] amino acids (17) were evaluated
`as dual inhibitors of NEP and ACE. Substitution
`with isosteres (-F, -OH, -NH2) as described by
`Grimm’s Hydride Displacement Law (Table 2) re-
`sulted in retention of activity. It was observed within
`this series, however, that the increase in the effective
`van der Waal’s radii of the isosteric substituents
`resulted in a decrease in activity (Table 9). In this
`instance, no significant alteration in preferential
`activity with either of the peptidases, ACE or NEP,
`was observed for these bioisosteres.
`The empirical approach used to advance the struc-
`ture-activity relationships with these peptidase
`
`+
`
`Patani and LaVoie
`
`Figure 11.
`Table 9. In Vitro Inhibition of NEP and ACE by
`N-[2-(Mercaptomethyl)-3-phenylbutanoyl] Amino
`Acids
`
`IC50 (nM)
`effective
`angiotensin
`van der Waal’s
`neutral
`converting
`radii (Å)31
`compound R
`endopeptidase
`enzyme
`17a
`2.5
`4.3
`1.20
`H
`17b
`5.9
`6.9
`1.47
`F
`17c
`9.0
`7.9
`1.53
`OH
`17d
`NH2
`1.79
`12.0
`16.0
`inhibitors is useful despite the fact that a more
`selective ACE inhibitor was not developed. Retention
`of activity within this series of bioisosteres permits
`an assessment of the validity of a possible correlation
`with one or more specific physicochemical param-
`eters. This study, for example, did provide insight
`into structural features which were critical to their
`activity as inhibitors of these peptidases.
`Recently, several 8-substituted O6-benzyl guanines
`(18, Figure 12) were evaluated for their ability to
`inactivate the human DNA repair protein, O6-alkyl-
`guanine-DNA alkyltransferase (AGT) (Table 10).32
`Inactivation of the human DNA repair protein O6-
`alkylguanine-DNA alkyltransferase by exposure to
`compounds such as O6-benzylguanine leads to a
`dramatic enhancement in the cytotoxic response of
`human tumor cells and tumor xenografts to chemo-
`therapeutic drugs. This effect is principally observed
`for chemotherapeutic agents whose mechanism of
`action involves modification of DNA guanine residues
`at the O6-position. In this study the effect of the
`interchange of NH2, OH, as well as CF3 (a bioisostere
`for a methyl group based on the replacement of
`hydrogen with fluorine) on activity was assessed.
`Analogues possessing electronegative groups at the
`8-position were more effective as inactivators of AGT
`in human HT29 colon tumor cell extracts. The
`relative activities of these bioisosteres based on the
`dose required for 50% inhibition (ED50) along with
`their electronegativities are outlined in Table 10.
`
`Figure 12.
`Table 10. Alkyl Guanine Transferase Inactivating
`Activity of 6-(Benzyloxy)purine Derivatives
`compound
`R
`electronegativity29
`ED50 ((cid:237)M)a
`18a
`NH2
`2.61
`2.0
`18b
`CF3
`3.46
`0.25
`18c
`OH
`3.51
`0.15
`a Effective dose required to produce 50% inactivation in HT-
`29 cells upon incubation for 4 h.
`
`LUPIN EX1046, Page 6
`
`

`
`+
`
`+
`
`Bioisosterism: A Rational Approach in Drug Design
`
`Chemical Reviews, 1996, Vol. 96, No. 8 3153
`
`Again, the retention of activity within a series of
`bioisosteres provides the basis for the discovery of a
`possible correlation between pharmacological activity
`and the physicochemical properties of specific agents.
`As an extension to the above group defined by
`Grimm’s Hydride Displacement Law, the widespread
`use of the chlorine atom as a bioisostere has been
`observed in several different series of biologically
`active compounds. This could be attributed to the
`similarity in size between these atoms, a comparison
`of which is made in Table 11. Further, there exists
`similarity in the lipophilicity of the methyl group
`with that of chlorine which may be responsible for
`its suitability as a monovalent bioisosteric replace-
`ment.
`N-(Substituted-3-pyridyl)-N ¢
`-alkylthioureas (19, Fig-
`ure 13), which have been evaluated as novel potas-
`sium channel openers,33 are among the more recent
`illustrations of the replacement of chlorine with
`isosteres from Grimm’s Hydride Displacement Law
`(Table 11). Potassium channel openers cause va-
`sorelaxation in vascular smooth muscle through
`hyperpolarization of the cell membrane. There is an
`increased interest in these compounds based on their
`therapeutic potential in the treatment of cardiovas-
`cular diseases. Substitution at the 6-position with
`monovalent isosteres (-NH2, -CH3, -Cl) results in
`analogues with similar biological activity.
`It was
`observed that substituents with similar biological
`activity had comparable effective van der Waal’s radii
`(Table 11). The methyl group, which has a lower
`electronegativity, elicited a weaker pharmacological
`response, suggesting an additional correlation be-
`tween activity and a physicochemical property.
`
`Figure 13.
`
`Table 11. Inhibition of Spontaneous Mechanical
`Activity in Rat Portal Vein (in vitro)
`maximum
`effective
`electro-
`fall in SBPa
`van der Waal’s
`radii31 (Å)
`negativity29
`(%)
`X
`compound
`19a
`1.79
`2.61
`29
`NH2
`19b
`CH3
`1.80
`2.27
`18
`19c
`1.73
`3.0
`27
`Cl
`a Antihypertensive activity measured as maximum % fall
`in systolic blood pressure in anesthetized normotensive rat by
`iv injection.
`
`Table 12 lists the relative potency of a group of
`bioisosteres which act as inhibitors of thymidylate
`synthase. Each of these benzo[f]quinazolin-1(2H)-
`ones (20, Figure 14), inhibit thymidylate synthase by
`virtue of their structural relation to its cofactor, 5,10-
`methylenetetrahydrofolic acid.34 They are, therefore,
`referred to as folate-based thymidylate synthase
`inhibitors. These analogues differ from other folate-
`based thymidylate synthase inhibitors as the absence
`of a glutamate residue suggests that they are not
`dependent upon active folate transport and poly-
`glutamylation for activity, the two mechanisms of
`resistance that have been observed with agents such
`
`Figure 14.
`
`Table 12. Thymidylate Synthase Enzyme Inhibition
`Data for Benzo[f]quinazolin-1(2H)-ones
`thymidylate synthase
`inhibitory activity
`IC50 ((cid:237)M)a
`X
`compound
`20a
`0.025
`Cl
`20b
`CH3
`0.178
`20c
`0.48
`OH
`20d
`NH2
`0.63
`20e
`1.08
`H
`a Inhibitor concentration producing 50% inhibition and
`determined by the tritium release assay of Roberts35 as
`modified by Dev et al.36
`
`as methotrexate. Within this series, it was observed
`that hydrogen bond donors were more potent than
`the unsubstituted parent compound. These ana-
`logues, however, were less active than compact
`lipophilic groups in elevating thymidylate synthase
`inhibition. Thus, at the 9-position, optimal size and
`lipophilicity appear to be critical factors associated
`with their ability to inhibit thymidylate synthase.
`In another study aimed at designing cholinergic
`agents which would be capable of penetrating the
`central nervous system and displaying high efficacy
`at the cortical muscarinic receptors, a series of
`oxadiazole-based tertiary amines 21 (Figure 15, Table
`13) were tested. The assay used was designed to
`measure affinity and predict cortical efficacy from the
`antagonist-agonist (i.e. NMS