Journal ofMedicinal Chemistvy
`
`© Copyright 1980 by the American Chemical Society
`
`Volume 23, Number 12
`
`December 1980
`
`Perspective
`
`Prodrugs and Site-Specific Drug Delivery
`
`Valentino J. Stella“
`
`Department of Pharmaceutical Chemistry
`
`and Kenneth J. Himmelstein
`
`Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, Kansas 66045.
`Received June 18, 1980
`
`PRO-
`‘ MOIETY
`
`DRUG
`
`ENZYMATIC
`OF-l
`NONENZYMATEC
`BIOTRANSFORMATION
`
`Scheme I
`
`1'
`
`PRO-
`MOIETY
`
`SYNTHESIS
`
`
`
`the technique could be used to temporarily alter and so
`optimize the physicochemical properties and, thus, the
`pharmacological and toxicological time profiles of an agent.
`Even before Albert, terms such as drug latentiation” and
`bioreversible derivatives were used by various investigators
`to describe such derivatives.
`As the area of biopharmaceutics and pharmacokinetics
`grew in the late 1960's and early 1970’s, knowledge and
`expertise was at last available that allowed deficiencies
`such as poor bioavailability to be identified in existing drug
`products and provided the basis for the better design of
`new products. Thus, the renewed interest in prodrugs was
`perhaps due to the growth of these disciplines, an increased
`understanding of metabolic processes in the body, and the
`perceived need to approach drug therapy and drug design
`more rationally.
`To date, much of the published work on prodrugs has
`focused upon what might be called “recla.mation“ projects.
`That is, the less than ideal behavior of a currently used
`therapeutic agent was traced to a particular physico-
`
`(5) N. J. Harper, J. Med. Pharm. Chem., 1, 467 (1959).
`(6) N. J. Harper, Prog. Drug Res., 4, 221 (1962).
`
`The term prodrug is used to describe an agent which
`must undergo chemical or enzymatic transformation to the
`active or parent drug after administration, so that the
`metabolic product or parent drug can subsequently exhibit
`the desired pharmacological response. The purpose of this
`paper is to address, in a critical and quantitative manner,
`whether prodrugs can provide site-specific delivery or
`targeting of parent, active drugs to their site of action. The
`first point that will be made is that prodrugs of most
`currently useful therapeutic agents cannot achieve further
`site-specific delivery. However, site-specific delivery is
`possible when drugs have certain physicochemical prop-
`erties. Thus, the thesis presented in this paper is that the
`physicochemical properties of the parent drug and the
`properties of the site are both critical in predicting whether
`a prodrug can succeed in site-specific delivery of the parent
`drug to that site. Drug design that is guided by such an
`analysis may be more successful in the development of
`targeted drug systems utilizing prodrugs.
`Although prodrugs have received renewed interest of
`late,” the approach is not new. Albert‘ was the first to
`use the term “pro-drug" or “pro-agent" and suggested that
`
`(2)
`
`(1) The authors’ work in this area have been supported by Re-
`search Grants from the National Institute of Neurological and
`Communicative Disorders and Stroke (NS 11998), National
`Institute of General Medical Sciences (GM 22357), Inter, Re-
`un .
`if-‘:al'(c31h Corp, and the University of Kansas General Research
`(a) T. Higuchi and V. Stella, in "Abstracts of Papers". 168th
`National Meeting of the American Chemical Society, Atlantic
`City, N.J., Sept. 10, 1974, American Chemical Society, Wash-
`ington, D.C., 1974, Abstr MEDI.
`(b) T. Higuchi and V. Stella,
`ACS Symp Ser., No. 14 (1975).
`(a) “Design of Biopharmaceutical Properties through Prodrugs
`and Analogs“. A symposium sponsored by the Medicinal
`Chemistry Section of the American Pharmaceutical Associa-
`tion, Academy of Pharmaceutical Sciences, 21st National
`Meeting, Orlando, FL, Nov 16-17, 1976.
`(b) “Design of Bio-
`pharmaceutical Properties through Prodrugs and Analogs", E.
`B. Roche, Ed., American Pharmaceutical Association, Wash-
`ington, D.C., 1977.
`(4) A. Albert, “Selective Toxicity", Chapman and Hall, London,
`1951.
`
`(3)
`
`0022-2623/SW1823-1275$0l.00/0
`
`© 1980 American Chemical Society
`
`Patent Owner, UCB Pharma GmbH — Exhibit 2049 - 0001
`
`

`
`1276
`
`Journal of Medicinal Chemistry, 1980. Vol. 23, No. 12
`
`Perspective
`
`Scheme II
`
`PRODRUG
`TARGET Tl$SUE. v,,,
`
`TARGET TISSUE. V1-D
`
` PARENT DRUG
`
`PRODRUG
`VOLUME or
`onsrmsuruow, V,
`
`PARENT DRUG
`VOLUME OF
`DlSTRIBUT|ON, VD
`
`
`
`pothetical model for a prodrug capable of permeating and
`releasing drug in a target organ is presented in Scheme II.
`The model assumes that the prodrug is introduced into
`the body as a dose, D, and distributes throughout a volume
`of distribution, Vp, and into the target organ of volume
`V1-P» with a clearance lap in mL / min. The prodrug is
`converted to the parent drug in the target organ or in the
`rest of the body via a saturable process described by a
`Michaelis—Menten form defined with K,,, and V,,,,,, values.
`The prodrug may be cleared via urinary excretion or
`nonproductive metabolism, kpel. The parent drug has a
`volume of distribution, VD, and a target organ of volume
`V1-D (equal to VTp). The transfer between the target organ
`and the rest of the body is defined by a clearance term,
`leg, while elimination from the body is defined by a
`clearance term, kD,,. The two input or transport terms,
`kP,,,,,,,, and kD,,,,,,,,, represent the possibility of direct or local
`delivery of prodrug or drug to the target site, respectively.
`To further simplify matters in the initial discussion, the
`assumptions that the prodrug will be administered sys-
`temically (kplml = kD,,,,,,,1 = 0 mL/min) and that the pro-
`drug quantitatively regenerates the parent drug (tape, = 0)
`will be made.
`An explanation of the term clearance (kn, kp, etc.) is in
`order. In classical pharmacokinetics, transport in and out
`of an organ has normally been expressed in terms of for-
`ward and reverse first-order rate constants. In this paper,
`clearance terms have been used instead of forward and
`reverse rate constants because of their physioiogical rele-
`vance. The rate at which a molecule can be transported
`to an organ is a function of two terms:
`the blood flow to
`the organ and the extraction coefficient of the organ, i.e.,
`
`kP=Q><E
`
`where hp is the clearance in milliliters/ minute, Q is the
`blood flow to the target organ in milliliters / minute, and
`E is the extraction coefficient or fraction extracted having
`the limits of 0 to 1. Poor transport to an organ can come
`from two sources. First, the physicochemical properties
`of the drug molecule in question may cause the molecule
`to be poorly permeable to some rate-limiting membrane,
`e.g., the blood—brain barrier. If this is the case, then E will
`be small and kp might be largely determined by the ex-
`tractability of the drug. On the other hand, if the drug
`readily permeates the organ (E ~ 1), then blood flow rate
`may become a limitation. Drug treatment of tumors may
`provide a good example of this dilemma,” because tumors
`have poor vascu1arization.13'19 Thus, simply trying to
`further increase membrane permeabilities of a drug for
`which E is approximately unity will have no effect on the
`ability of the drug to reach the target site, since the
`rate-determining step is blood flow, not extractability.
`Therefore, using the model and clearance concepts de-
`scribed above, it is possible to predict the maximum values
`for k]: for particular organs if blood-flow rate to the organ
`is known. Lower values than Q might be predicted for kp
`if the permeability of a rate-limiting membrane for the
`organ in question is known.
`By setting up mass balance equations for each substance
`in each tissue it is possible to generate a series of differ-
`ential equations which can then be solved numerically.’-'0
`A typical mass balance equation for the parent drug in the
`target tissue is shown in eq 1 and 2, where the various C
`
`(17) P. Workman and J. A. Double, Biomedicine, 28, 255 (1978).
`(18) L. H. Gray, A. D. Conger, M. Ebert, S. Hornsey, and 0. C. A.
`Scott, Br. J. Radiol., 26, 638 (1953).
`(19) R. H. Thomlinson and L. H. Gray, Br. J. Cancer. 9, 539 (1955).
`(20) By use of a modified I-Iamming’s predictor-corrector method.
`
`Patent Owner, UCB Pharma GmbH — Exhibit 2049 - 0002
`
`
`
`DOSE
`
`EL|MlNATlON
`
`chemical property of that agent. To overcome this limi-
`tation, prodrug forms of the agent or other techniques were
`considered to correct the problem.
`Prodrugs Defined on the Basis of Their Problem-
`Solving Potential. The prodrug approach to problem
`solving is illustrated in Scheme 1. When the parent or
`active drug is not fully utilized because of some identifiable
`barrier or problem,” the physicochemical properties of the
`drug can be altered by attachment of a pro-moiety. This
`allows the prodrug to bypass the barrier and, once past the
`barrier, to revert to the parent compound by a postbarrier
`enzyme or nonenzymatic process. An alternative to
`cleavage as a method for obtaining activation is enzyme-
`mediated synthetic processes such as phosphorylation.
`Other literature reviews provide many examples of
`where prodrugs have been used to solve various prob-
`lems?” These reviews should be consulted for a more
`extensive coverage of the subject. What will be presented
`in this perspective will be some thoughts on one particular
`direction for future research with prodrugs, i.e., use of
`prodrugs for site-specific delivery or targeting of recep-
`tor-active chemical entities.
`Site-Specific Delivery. To achieve truly site-specific
`delivery, the time profile of drug at the target organ must
`be optimized, and the burden of drug to other tissues must
`be minimized. One way to visualize how to use prodrugs
`for optimizing drug delivery to a particular site would be
`to develop a hybrid classicalf physiologically based phar-
`macokinetic model in which the various hypothesized op-
`timization prodrug techniques are tested. A simple hy-
`
`in “Pharmacology and
`('i') E. J. Ariens and A. M. Simonis,
`Pharmacokinetics", T. Toerell, R. L. Dedrick, and P. G.
`Condliffe, Eds., Plenum Press, New York. 1974, p 163.
`(S) V. J. Stella, T. J. Mikkelson, and J. D. Pipkin, in “Drug De-
`livery Systems: Characteristics and Biomedical Applications“,
`R. L. Juliano, Ed., Oxford University Press, New York, 1980.
`Chapter 4.
`(9) A. Albert, “Selective Toxicity", 5th ed, Chapman and Hall,
`London, 1973, pp 21-62.
`(10) E. J. Ariens, Prog. Drug Res., 10. 629 (1966).
`(11) E. J. Ariens in “Drug Design", Vol. 2, E. J. Ariens, Ed., Aca-
`demic Press, New York. 1971, pp 2-127.
`(12) G. A. Digenis and J. V. Swintosky, Hcmdb. Exp. Pharmocot,
`28(Part 3), 36 (1975).
`(13) A. A. Sinkula and S. H. Yalkowsky, J. Pharm. Sci., 64, 181
`(1975).
`(14) A. A. Sinkula, Annu. Rep. Med. Chem., 10, 306 (1975).
`(15) V. J. Stella, Aust. J. Pharm. Sci., NS2, 57 (1973).
`(16) V. J. Stella in “Formulation and Preparation of Dosage
`Forms”, J. Polderman, Ed., Elsevier/North-Holland, Amster-
`dam, l9'T7, pp 91-111.
`
`

`
`Journal of Medicinal Chemistry, 1980, Vol. 23, No. 12
`8
`
`1277
`
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`
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`DRUGCONCENTRATIONINTARGET0RGAN.HQlmI
`
`Perspective
`
`rate of change of parent drug into the target organ =
`net rate in (by transport) +
`rate in (by metabolism of the prodrug) (1)
`
`Vrn
`
`dC'I‘D
`dt
`
`Vma.x’CTP
`— kn(Cn ‘ Crnl + KT"":+CT?
`
`(2)
`
`terms represent concentrations of drug as defined by the
`subscripts. The Vm,’ term in eq 2 has the units of mass
`of prodrug metabolized per unit time. To convert this to
`the relative enzymatic activity based on a per unit volume
`of tissue it must be divided by V-pp. Equal enzymatic
`activity on a per volume basis in the target tissue relative
`to the rest of the body occurs when Vm,,’/ VT? = muf Vp.
`Values can then be given to each of the parameters in these
`equations and concentration—tin1e profiles of prodrug and
`drug in the two tissues generated. By fixing all parameters
`except for the one to be probed, the effect of parameter
`change on the time profile of each species can be examined.
`In the simulations (see Figures 1-5 later), the relative
`availability of a drug to a particular organ can be deter-
`mined by the area under the target organ concentration
`vs. time profile (AUC) of the formed parent drug. Con-
`centration of drug at one particular time point might be
`deceiving in its interpretation in that it may not be rep-
`resentative of the differences over the entire time course
`
`of target organ concentration (see Figure 3 later).
`Why many of the prodrug approaches to solving drug
`site-specific delivery problems have in the past met with
`limited success can be examined by the use of this model.
`Membrane Permeability Alterations and Site-Spe-
`cific Conversion. Consider the idea that permeability
`of a membrane is the rate-limiting step to a drug’s ability
`to reach the active site. Creveling et al.” and Daly et al.”
`have demonstrated increased permeability of nor-
`epinephrine derivatives to the brain (3,4,,6’-triacetyl and
`3,4,8-trimethylsilyl derivatives). These proposed prodrugs
`enter the brain much more readily than does the polar
`parent drug, norepinephrine. However, the derivatives
`survive in the brain largely as noncatechol entities. The
`norepinephrine prodrugs are able to reach the site, but
`their inability to convert to the parent drug in the target
`tissue simply cause the prodrug to drain from the target
`site (i.e., kp >> kg but Vmu’/V-1-P <<< Vmuf VP). There-
`fore, using increased permeability as the only basis for
`judging improvement in drug delivery via prodrugs may
`be an unacceptable or limited criterion for specificity.
`An alternative criterion for specificity can be based upon
`the target organ containing a high level of a particular
`enzyme which is capable of selectively cleaving the pro-
`moiety—drug linkage at that site (V,,,,,,’/ V-pp >> Vm,/ VP).
`This argument appears promising, but it also suffers from
`narrow thinking.
`It has been proposed that the higher
`concentration of phosphatases and amidases in tumor cells
`could be used to site specific deliver cytotoxic agents to
`tumors. In fact, diethylstilbesterol diphosphate has been
`promoted as a human prostatic tumor-selective agent,25
`as have other phosphate ester derivatives.” Again, apart
`
`(21) C. R. Creveling, J. W. Daly, T. Tokuyama. and B. Witkop.
`Experientia, 25, 26 (1969).
`(22) J. W. Daly, C. R. Creveling, and B. Witkop, J. Med. Chem., 9,
`273 (1966).
`(23) H. Druckrey and S. Raabe, Klin. Wochen.schr., 30, 882 (1952).
`(24) For an excellent discussion of this point and drug latentiation
`in cancer chemotherapy in general, please refer to ref 17 and
`the references therein.
`(25) P. Bey. M. Jung, and B. Metcalf, Med. Chem., Proc. Int.
`Symp., 5th, 1976, 115 (1977).
`(26) P. Bey, Sci. Tech. Pharm., 7, 171 (1975).
`
`0
`
`I
`
`2
`
`3
`
`4
`HOURS
`
`5
`
`Figure l. Plots of the effect of varying kp values, as defined by
`a hypothetical prodrug model (Scheme II), on drug concentration
`in the target organ vs. time profile for a drug having a lap value
`of 20 mL/min and prodrugs where Vm’/ V-I-P = 10(V,,,_,,/ Vp). Line
`1 is where drug input is, as the parent drug, placed in volume VD.
`Lines 2 and 3 are where drug input is, as prodrugs, placed in
`volume V; and having a 12,: value of 0.02 and 10 mL/min, re-
`spectively.
`
`from other considerations that will be discussed later, these
`phosphate esters were only partially successful because the
`prodrug that was designed to convert to the parent com-
`pound in a specific tissue was not able to reach that tissue.
`The ubiquitous distribution of phosphatases in other tis-
`sues more highly perfused and accessible to the prodrug,
`such as bone marrow, small intestines, and liver,“ are
`probably able to compete more effectively for the cleavage
`of the prodrug. Referring to Scheme II, V,,,,,’/
`V1-p>> V,,,,,/ VP, but kn » kp. This is probably the reason
`why many attempts using peptidases, glycosidases, sulfa-
`tases, and phosphatase enzymes to promote tumor selec-
`tivity of cytotoxic agents have failed in the past.“ Pro-
`drugs attempting to use these enzymes are too polar, the
`relative enzymatic selectivity is insufficient, and tumor cell
`perfusion is too poor to achieve the desired goal.
`Another example of poor prodrug transport is ~/-vinyl-
`—yAbu (1), a 7-aminobutyric acid (w/Abu) transamlnase km,
`
`9
`H,N
`
`pH=cHz
`ca
`\CH2
`I
`
`ca,
`
`/
`
`\coo°
`
`inhibitc-r.25‘27 In the case of 7-vinyl--yAbu, its purpose is
`to selectively inhibit the -yAbu transaminase enzyme to
`raise synaptasomal ~yAbu levels, which should then lead
`to anticonvulsant action. Gale and Iadarol.a"3 have recently
`shown that intraperitoneal (ip) injection of 7-vinyl--yAbu
`at 1600 mg] kg to rate does act as an anticonvulsant.
`However, the high dose needed to elicit the response
`suggests that very little 7-vinyl-yAbu penetrates the
`blood—brain barrier. The need for improved delivery of
`~yAbu/ glutamate altering agents has been recognized, and
`
`(27) M. Jung and B. Metcalf, Biochem. Biophys. Res. Commun., 67,
`301 (1975).
`(28) K. Gale and M. J. Iadarola, Science, 203, 288 (1980).
`
`Patent Owner, UCB Pharma GmbH — Exhibit 2049 - 0003
`
`

`
`1278
`
`Journal of Medicinal Chemistry, 1980, Vol. 23, Na. 12
`
`Perspective
`
`Fglml
`DRUGCONCENTRATIONINTARGETORGAN.
`
`
`
`
`HOURS
`
`Figure 2. Plots of the effect of varying kp values, as defined by
`a hypothetical prodrug model (Scheme II), on drug concentration
`in the target organ vs. time profile for a drug having a kn value
`of 200 mL/min and prodrugs where V_,’/ V-pp = 10(V,,,,,,/ Vp).
`Line 1 is where drug input is, as the parent drug, placed i.n volume
`VD and line 2 is where drug input is, as a prodrug, placed in volume
`Vp and having a lap value of 200 mL/min.
`
`‘uU’‘A
`E 30
`2'4
`
`U s E
`
`0Di
`<I-
`
`20
`
`3
`
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`
`E
`2
`
`9 :
`
`10I
`
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`
`D L
`
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`
`1
`
`2
`
`3
`
`4
`HOURS
`
`5
`
`6
`
`T
`
`8
`
`Figure 3. Plots of the effect of varying kp values, as defined by
`a hypothetical prodrug model (Scheme II), on drug concentration
`in the target organ vs. time profile for a drug having a kD value
`of 20 mL/min and prodrugs where V,,,,,’/ V-pp = 10(V,,,_,/ VP). Line
`1 is where drug input is, as the parent drug, placed in volume VD.
`Lines 2 and 3 are where drug input is, as prodrugs, placed in
`volume Vp and having kp values of 20 and 200 mL/ min, re-
`spectively.
`
`target organ, even though a large fraction of the parent
`drug is formed from the prodmg by metabolism specifically
`in the target organ.
`One example of the problem of rapid “leakage“ of the
`formed drug from the target tissue may be 'y-glutamy1-
`sulfamethazole (2), a proposed kidney selection prodrug
`of sulfamethazole (3).3° This attempted delivery of sul-
`famethazole is based on earlier studies showing the rela-
`tively high kidney activity of the ‘y-glutamyltranspeptidase
`
`prodrugs of such km, inhibitors (which themselves can be
`considered prodrugsmzfil and other ’yAbu/glutamate al-
`tering agents having better blood—brain barrier permea-
`bility have been proposed.”
`Illustration of the above cases is possible using the model
`presented in Scheme II (Figure 1).
`It is assumed for all
`of the simulations (Figures 1—5) that V-I-P = V1-D = 100 mL,
`Vp = VD = 14000 mL, dose = D = 100 mg, Vm, = 10
`mg/min, K,,,, = 1.2 ,u.g/mL, and kD,,1= 40 mL/min. For this
`simulation, kn is fixed at 20 mL/min, which represents a
`clearance value similar to that of a small tumor where the
`drug readily permeates the tissue. These conditions are
`then used to input the parent drug (equivalent to an iv
`dose of the parent drug into VD) to generate a target organ
`drug concentration as a function of time. This profile is
`then used as a base line for comparison with the profile
`generated from the input of a prodrug. The conditions Km’
`= Km, hp = 0.02, 10 mL/min and Vm,’ = 0.714 mg/min
`(giving a specific activity on a per volume basis ten times
`higher in the target organ than in the rest of the body, i.e.,
`Vm,’/ VT? = 10 V,,,,,,,/ VP), for the prodrug describes a
`prodrug that is rapidly metabolized by the target organ
`but has lower permeability
`to the target organ than the
`parent drug. Figure 1 illustrates that the superior target
`organ metabolism cannot compensate for the decreased
`availability to the site. In fact, input of the parent drug
`(line 1) gives more rapid drug input to the target organ
`than did the prodrug (lines 2 and 3).
`To summarize the discussion so far, altered permeability
`and selective enzymatic cleavage of prodrugs, although
`important in achieving targeting, cannot be treated as
`mutually independent factors. As will be demonstrated
`later, it is possible to trade one factor against another.
`However the degree of success of such a trade off depends
`upon another consideration which has not, until now, been
`fully recognized or discussed.
`If the
`The Properties of the Parent Compound.
`parent drug molecule reasonably permeates the target
`organ (note: specificity is not implied by this statement,
`it simply states that the time profile of drug in the target
`organ approximately mimics the plasma level time profile,
`kn 2 kD,,1), increased relative permeability by the prodrug
`and its specific conversion in the target tissue may do little
`to promote specificity. Figures 2 and 3 are simulations of
`two drugs where V-pp, V-1-D, Vp, VD, kD,,1, V3,”, and Km are
`the same as in Figure 1. In Figure 2, kg = 200 mL/ min,
`V,,,,,,,,’/ V-pp = 10(V,,,,,,/Vp), and kp = kg; in Figure 3, kg =
`20 mL/min, V,,,,,,’/V-rp = 10(V,,,,,,/ VP), and k1, = kn and
`kp = 105213. The values for kg of 200 and 20 mL/min
`represent cases of very rapid and moderate accessibility
`and retention of the parent drug to the target tissue, re-
`spectively.
`With the kn values of 200 mL / min, no real advantage
`via prodrug input is seen even with good permeability by
`the prodrug and a tenfold selectivity in target site con-
`version vs. body conversion.
`In fact, drug input via the
`prodrug delays the appearance of parent drug in the target
`tissue. With the kn value of 20 mL/ min (Figure 3) some
`increase in the early drug concentration time points is seen
`in the target organ, but the overall advantage of prodrug
`vs. parent drug input is minor. The only real advantage
`seen is that parent drug input via prodrug delivery pro-
`vides an alternative target organ input mode for the parent
`drug. The lack of specificity illustrated in Figures 2 and
`3 is due to rapid “leakage" of the parent drug from the
`
`(29) L. Brehrn, P. Krogsgaard-Larsen, and P. Jacobsen, Alfred
`Benzoin Symp., 12, 247 (1979).
`
`(30) M. Orlowslri, H. Mizoguchi, and S. Wilk, J. Pharmacol. Exp.
`Them, 212, 167 (1980).
`
`Patent Owner, UCB Pharma GmbH — Exhibit 2049 - 0004
`
`

`
`Journal of Medicinal Chemistry, 1980, Vol. 23, No. 12
`BO
`
`1279
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`‘-1 O
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`II D
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`DRUGCONCENTRATIONINTARGETCRGAN.IIQ/ml
`
`Perspective
`
`Q
`
`c009
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`i
`ll?
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`H3N-CH-CHz-CHz-C--1-- NH ~ -IS|—NH
`I
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`E
`
`I
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`
`Y -GLUTAMYLTFIANSPE PTIDASE
`i
`
`GLUTAMIC ACID
`
`4' HIN
`
`‘P
`%-NH
`0
`3
`
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`
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`N“O
`
`CH5
`
`enzyme. Will: et al. had previously used this kidney se-
`lectivity to deliver L-Dope. (L-3,4-dihydroxyphenylalanine)
`and, subsequently, dopamine as 7-glutamyl-L-Dopa to the
`kidney.“
`The high kidney -y-glutamyltranspeptidase
`activity relative to activity in other tissues serves as the
`basis for the potential selective renal delivery of sulfa-
`methazole via 7-glutamylsulfamethazole.
`The relative sulfamethazole concentrations in mouse
`kidneys and other tissues 20 min after ip injection of
`equimolar doses of sulfamethazole and 7-glutamylsulfw
`methazole were measured by Orlowski et al.3° Their data
`suggest that sulfamethazole concentration in tissues other
`than the kidney and pancreas are slightly diminished, but
`no great selectivity is seen in the kidney and pancreas.
`Other derivatives such as N-acetyl-7-glutamylsulfameth
`azole and N-(chloroacetyl)--y-glutamylsulfamethazole are
`more encouraging not because they give high levels of
`sulfamethazole in the kidney but because they do appear
`to give significantly diminished levels of sulfamethazole
`in other tissues.” Orlowski et al.3° have addressed some
`of the possible explanations for the poor behavior of 7-
`glutamylsulfamethazole. However, another possible ex-
`planation not addressed by those authors is that the rate
`of cleavage of 7-glutamylsulfamethazole to sulfamethazole
`at the specific site of cleavage allows the sulfamethazole
`to “leak" from the metabolism site. This leakage and
`redistribution may occur because sulfamethazole is a
`relatively nonpolar uncharged species at physiological pH.
`The other derivatives may cleave at sites where trapping
`of the sulfamethazole is possible.
`A leakage theory gains credibility and validity when the
`behavior of -y-glutarnylsulfamethazole is compared to that
`of 7-glutamyldopamine (4) and 7-glutamyl-L-Dopa (5).
`Wilk et al.“ and others32‘3‘ have shown that the ~y-gluta-
`myltranspeptidase activity in the kidney can be used to
`
`(31) S. Wilk, H. Mizoguchi, and M. Orlowski, J. Pharmocol. Exp.
`Them, 206, 227 (1978).
`(32) J. J. Kyncl, F. N. Minard, and P. H. Jones, Adv. Bioscil, 20.
`369 (1979).
`(33) J. Kyncl, R. Hollinger, R. Warner, C. W. Ours, F. N. Minard,
`P. H. Jones, and J. H. Bisl, Kidney Int., 10, 589 (1976).
`(34) J. Kyncl, R. Hollinger, C. W. Ours, F. N. Minard, P. H. Jones,
`and J. H. Biel,
`in “Abstracts of Papers", 172nd National
`Meeting of the American Chemical Society, San Francisco,
`Calif., 1976, American Chemical Society, Washington, D.C.,
`1976, Abstr MEDI I9.
`(35) P. H. Jones, C. W. Ours, J. H. Biel, F. N. Mina:-cl, J. Kyncl, and
`Y. C. Martin,
`in “Abstracts of Papers", 172nd National
`Meeting of the American Chemical Society, San Francisco,
`Calif., 1976, American Chemical Society, Washington, D.C.,
`1976, Abstr MED] 1'7.
`(36) F. N. Minard, J. C. Cain, D. S. Grant, C. W. Ours, J. Kyncl,
`P. H. Jones, and J. H. Biel, in "Abstracts of Papers”, 1’T2nd
`National Meeting of the American Chemical Society. San
`Francisco, Calif., 1976, American Chemical Society, WashiJig—
`ton, D.C., 1976. Abstr MED] 18.
`
`HOURS
`
`Figure 4. Plots of the effect of varying 12,. values, as defined by
`a hypothetical prodrug model (Scheme II), on drug concentration
`in the target organ vs. time profile for a drug having a kg value
`of 0.2 mL,’ min and prodrugs where V,,,,,’,’ V“: = 100(V,m/ Vp).
`Line 1 is where drug input is, as the parent drug, placed in volume
`VD. Lines 2, 3, and 4 are where drug input is, as prodrugs, placed
`in volume Vp and having is,» values of 0.2, 20, and 200 mL/min,
`respectively.
`
`selectively deliver dopamine (6) as 7-glutamyldopamine
`(4) or 7-glutamyl-L-Dopa (5). The 7-glutamyl-L-Dope
`releases L-Dopa (7), which then decarboxylates to dop-
`amine.
`
`J
`I
`II
`9 cooe
`o
`I
`is
`0"‘
`H,~-cs-ca,-cH,—c ——w~— NH-CH-CH2- -OH
`I
`Y- GLUTAMYLTRANSPEPT IDASE
`
`1.
`
`Flfi i.
`
`a=—coo9
`
`L-DOPA
`EDECARBOXYLASE
`
`.‘:
`
`1
`
`Both 7-glutamyldoparnine and 7-gultamyl-L-Dopa are
`as polar as 7-glutsmylsulfamethazole, but the released
`parent drugs, dopamine and L—Dopa, are very polar and
`charged at physiological pH. Both -y-glutamyldopamine
`and 7-glutamyl-L-Dopa have been found to be superior
`kidney delivery forms of dopamine relative to dopamine
`and L-Dopa themselves, as measured by kidney and other
`tissue level time profiles and pharmacological activity
`measurements."“'3°
`Figure 4 presents a simulation of the importance of
`parent drug retention. The parameters V-pp, V-m, Vp, VD,
`kD,;, Va“, and K,,, are the same as in Figure 1. The
`transport constant hp has been given a low value of 0.2
`mL/ min to represent a drug that has poor transport
`characteristics into and out of the target organ. This
`characteristic has been combined with selective cleavage
`of the proclrug in the target organ [Vm,,,,’/ V»;-P = 100-
`(V,,,,,/ Vp)] and varying degrees of transport of the prodrug
`to the target organ (hp = 0.2 to 200 mL/min). Note that
`kp values greater than kn values can only occur when the
`limitation is organ extractability, not blood perfusion. If
`the low value of kg actually represents a blood-flow lim-
`itation (E for the parent drug is approximately unity), then
`kp cannot have values greater than kn. As seen in Figure
`4, a substantial improvement in selectivity can be achieved
`
`Patent Owner, UCB Pharma GmbH — Exhibit 2049 - 0005
`
`

`
`1230
`
`Journal of Medicinal Chemistry, 1980, Vol. 23, N0. 12
`
`Perspective
`
`when all the conditions of transport, selective cleavage, and
`retention are operative. It should be noted that, while the
`enzymatic specificity used here is high, similar results are
`achieved with lower values of Vmu’.
`L-Dopa itself is an example of a prodrug of dopamine.
`L-Dopa is able to deliver dopamine to the brain, because
`it is transported to the brain via the active—transport
`mechanism for L-amino acids.” Once in the brain, it is
`subsequently decarboxylated to dopamine. As a prodrug
`of dopamine, L-Dopa is not without problems. Peripheral
`decarboxylation of L-Dopa to dopamine leads to various
`side effects that are directly attributed to peripheral do-
`pamine and its further metabolites. Selectivity for brain
`delivery is partially achieved by use of the peripheral L-
`Dopa decarboxylase inhibitors. That is, the combination
`of L-Dopa and a peripheral Dopa decarboxylase inhibitor
`help build selectivity into the delivery of dopamine to the
`rain.
`Another possible example of the importance of site
`permeability and retention is the antiviral agent acyclovir
`(8), which is selectively activated to its phosphate deriv-
`ll
`
`ative (9) in viruses. Acyclovir is an analogue of a vital
`nucleotide precursor; it is not phosphorylated by mam-
`malian cells but is phosphorylated by the viral enzyme.
`The active agent 9 is then incorporated into viral DNA,
`disrupting the virus’s replication cycle.” The selectivity
`in this case may come not only from the activation process
`(phosphorylation of 8 —* 9 in viruses) but also from 8 being
`able to penetrate the virus and from 9 probably being
`retained by the virus. If 9 is partially released from the
`virus, it probably would have difficulty being taken up by
`mammalian cells because of its high polarity. To date, 8
`has shown low toxicity in man in phase I studies, and its
`possible use in the treatment of herpes infections can be
`a major breakthrough.”
`An additional example of a site-specific delivery stressing
`the importance of the physicochemical properties of the
`parent drug is thiamine tetrahydrofurfuryl disulfide or
`TTFD (10), a lipid-soluble prodrug of thiamine (11).5'“‘*‘°
`
`l
`
`CH3
` CHz\N
`H,,c ‘N
`NH bfiq/“3Hz‘C
`L
`H
`H2-OH
`S-S-CH2-E0]
`Q
`I
`GLUTATHi0NE
`
`2
`
`94/
`a '
`”
`
`“:9
`
`°"'2-~..fi
`ll
`5
`
`N"?
`
`ll
`
`CH,
`
`CH2-CH2-OH
`
`(37) H. Shindo, T. Komai, and K. Kawai, Chem. Pharm. Br.rll., 25,
`1417 (1977).
`(38) J. A. Fyfe. P. M. Keller, P. A. Furman, R. L. Miller, and G. B.
`Elion, J. Biol. Chem, 253, 8721 (1978).
`(39) G. B. Elion, Chem. Eng. News, 58(15), 24 (1980).
`
`pg/ml
`DRUGCONCENTRATIONINTARGETORGAN,
`
`
`
`HOURS
`
`Figure 5. Plots of the effect of varying hp values, as defined by
`a hypothetical prodrug model (Scheme II), on drug concentration
`in the target organ vs. time profile for a drug having a kg value
`of 0.02 mL,/ min and prodrugs where V,,,_,’/ V“: = mu] VP. Line
`1 is where drug input is, as the parent drug, placed in volume VP.
`Lines 2, 3, 4, 5, and 6 are where drug input is, as prodrugs, placed
`in volume Vp and having hp values of 0.02, 0.2, 2, 20, and 200
`mL/min, respectively.
`
`After intravenous (iv) administration of thiamine to rats,
`thiamine is rapidly cleared primarily via urinary excretion
`(90% dose) from whole blood with a half-life of 35 min.
`The thiamine in this case is the plasma fraction of the
`whole blood. After iv administration of TTFD, whole-
`blood thiamine half-life is 200 min (all the thiamine is in
`the red blood cell component of the whole blood) and 76%
`of the administered dose can be accounted for in the red
`blood cell fraction within minutes after administration of
`the TTFD.“ The long half-life of thiamine from whole
`blood results from TTFD rapidly and passively permeating
`the red blood cell membranes and reacting instantaneously
`with red blood glutathione thus releasing thiamine. The
`release of the trapped thiamine from the red blood cells
`is slow at these levels,‘°"“ and the long whole-blood half-life
`actually represents the slow passive permeation of the
`quaternary thiamine through the red blood cell mem-
`brane.“° Although the initial goal had not been the delivery
`of thiamine to the red blood cells, this work did show that
`a considerable fraction of a drug can be delivered to an
`individual tissue if the right conditions are met.
`The cases discussed above all assume enzymatic spe-
`cificity of the target organ over the rest of the body.
`However, improved drug delivery may be possible to a
`specific site even without specificity of the enzymatic
`process when the parent drug in question has difficulty
`reaching the desired site. Figure 5 illustrates this case.
`Again, VTp, V-1-D, Vp, V13, 5291, Vm, and km are the same as
`in Figure 1