`
`BUPREN‘ORPHINE:
`
`COMBATTING DRUG ABUSE
`WITH A UNIQUE OPIOID
`
`
`Editors
`
`ALAN COWAN
`
`Department of Pharmacology
`Temple University School of Medicine
`Philadelphia, Pennsylvania
`
`
`
`JOHN W. LEWIS
`
`School of Chemistry
`University of Bristol
`Bristol, England
`
`@WILEY-Liss
`A IOHN WILEY & SONS, INC. , PUBLICATION
`New York ' Chichester - Brisbane 0 Toronto 0 Singapore
`
`I
`
`RB EX. 2027
`BDSI V. RB PHARMACEUTICALS LTD
`IPR2014-00325
`
`Page
`
`1
`
`Page 1
`
`RB Ex. 2027
`BDSI v. RB PHARMACEUTICALS LTD
`IPR2014-00325
`
`

`

`
`
`Richard B.
` The text of this book is printed on acid-free paper.
`
`Contrib '
`
`Forewor
`
`George E.
`
`Preface
`
`PRECLI
`
`' Buprenor‘
`
`Update o"
`Alan Cowa‘il
`
`Behaviorfi
`Buprenor]
`Linda A. D}
`i
`Reinforciii
`and Phy
`
`
`
`Page 2
`
`5:
`
`:MJ
`
`'rt:err-en’s
`a“A?“
`
`Address All Inquiries to the Publisher
`Wiley-Liss, Inc., 605 Third Avenue, New York, NY 10158-0012
`
`Copyright © 1995 Wiley-Liss, Inc.
`
`Printed in the United States of America.
`
`Under the conditions stated below the owner of copyright for this book hereby grants
`permission to users to make photocopy reproductions of any part or all of its contents for
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`Law. This consent does not extend to other kinds of copying, such as copying for general
`distribution. for advertising or promotional purposes. for creating new collective works, or
`for resale.
`
`While the authors, editors, and publisher believe that drug selection and dosage and the
`specifications and usage of equipment and devices. as set forth in this book, are in accord
`with current recommendations and practice at the time of publication, they accept no legal
`responsibility for any errors or omissions, and make no warranty, express or implied, with
`respect to material contained herein. in View of ongoing research, equipment modifications,
`changes in governmental regulations and the constant flow of information relating to drug
`therapy, drug reactions and the use of equipment and devices, the reader is urged to review
`and evaluate the information provided in the package insert or instructions for each drug,
`piece of equipment or device for, among other things, any changes in the instructions or
`indications of dosage or usage and for added warnings and precautions.
`
`Library of Congress Cataloging-in-Publication Data
`
`2. Narcotic Dependence—
`
`Buprenorphine : combatting drug abuse with a unique opioid / edited by
`Alan Cowan and John W. Lewis.
`p.
`cm.
`Includes bibliographical references and index.
`.
`ISBN 0-471-56198-3
`2. Buprenorphine—Therapeutic use.
`l. Opioid habit—Chemotherapy.
`I. Cowan, Alan. 1942—
`.
`11. Lewis John W.
`[DNLM:
`l. Buprenorphine—therapeutic use.
`therapy. QV 92 B9443
`I994]
`RC568.058387
`1994
`616.86'32061 —dc20
`DNLM/DIE
`
`
`
`for Library of Congress
`
`94-28470
`ClP
`
`lH l H“ HillWilli“lllilli
`
`Allull 652L735-
`
`l
`
`Page 2
`
`

`

`ABSORPTION, DISTRIBUTION,
`METABOLISM, AND EXCRETION
`OF BUPRENORPHINE IN
`ANIMALS AND HUMANS
`
`
`
`--v'@NwAVv'lfiv‘o...
`
`“mammoth-.5.“5...,“
`
`This chapter reviews ADME (absorption, distribution, metabolism, and excretion)
`studies carried out with buprenorphine in animals and humans. The drug was
`developed in the early 19708 by the pharmaceutical research and development
`departments of Reckitt & Colman Products, Ltd, UK, leading to its registration in
`the UK as an analgesic for moderate to severe pain in 1977 (Temgesic Injection®
`and Temgesic Sublingual® tablets). Since then the products have been registered in
`over 40 countries. The main findings of ADME studies were summarized in an
`early review by Heel et al. [1979].
`Drug metabolism studies were made difficult because of the high potency of
`buprenorphine such that at normal therapeutic doses chromatographic techniques
`were pushed to the limits of sensitivity for measuring plasma and tissue levels of the
`drug. Much work was carried out to provide a specific and sensitive chemical assay
`for the drug, with progressive use of gas chromatography (GC), high-performance
`. liquid chromatography (HPLC), and GC/mass spectroscopy. Good, reproducible
`linear assays have been produced by all three methods, but sensitivity has always
`been a problem with GC and HPLC and sample throughput is very slow with
`GC/MS. Parallel development of a radioimmunoassay for buprenorphine gave two
`antibodies that bind with buprenorphine. Unfortunately, one of the antibodies cross-
`
`DONALD S. WALTER
`Reckitt & Colman Products, Hull HUB 7DS, UK
`
`' CHARLES E. INTURRISI
`Department of Pharmacology, Cornell University Medical College, New York, NY
`10021
`
`INTRODUCTION
`
`Buprenarphine: Combatting Drug Abuse Willi a Unique Opioid, pages 113—135
`© I995 Wiley-Liss, Inc.
`
`Page 3
`
`

`

`3
`
`i
`
`E
`
`. l
`
`i
`
`:
`
` i.
`
`4.
`
`a.'rrw-a
`
`
`
`114
`
`WALTER AND INTURFlISl
`
`'
`
`,
`
`NCH2—<]
`
`
`
`
`
`
`
`
`f,
`;
`
`
`
`
`HO
`
`,
`
`0'
`
`(CH3
`-C£\-C(CH3)3
`OH
`OCH3
`
`Buprenorphine
`
`NH
`
`Q -C’\—-C(CH3)5
`HO
`0
`OCH3
`
`,3 H3
`
`OH
`
`‘1
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`'
`
`
`
`
`i‘i‘l‘fl‘ifivn'
`
`N-Dealkylbuprenorphine
`
`Fig. 1. Structures ofbuprenorphine and N-dealkyl buprenorphine.
`
`l) and the other
`reacted with a major metabolite, N-dealkyl buprenorphine (Fig.
`antibody cross—reacted with the other major metabolite, buprenorphine glucoronide
`conjugate. However, radioimmunoassay with the first antibody has been used exten-
`sively in clinical research and human bioavailability studies because this was the
`only feasible assay for providing information about
`the absorption and phar-
`macokinetics of the drug in humans. Its use in single-dose studies is justified
`because the contribution of the N-dealkyl metabolite to total immunoreactivity after
`a single dose is very low; this is discussed later.
`Drug metabolism studies are facilitated if a high—specific-activity, stable radio-
`labeled form of
`the drug is available. Various options
`for
`radiolabeling
`buprenorphine were considered; the most satisfactory option was a 14C label but the
`nonavailability of high-specific-activity carbon—labeled precursors and the poor
`yield in synthetic steps precluded the production of carbon-labeled buprenorphine.
`Labeling with 1251 has been successfully used for the radioimmunoassay [Hand et
`al., 1986] but this molecule was considered to be too dissimilar to the parent drug
`for ADME studies.
`
`A method for tritium labeling of buprenorphine to high specific activity was
`developed using tritium exchange at the 15,16-positions of buprenorphine [Rance et
`al., 1976]. The suitability of tritium-labeled buprenorphine was assessed by exam—
`ining lability of the label in rats over a period of 48 hr after a single intramuscular
`
`-
`
`Page 4
`
`Page 4
`
`

`

`dose of drug [Brewster et al., 1981a]. Recovery of radioactivity in urine, feces,
`i‘carcass, and expired air was determined, The lability in each sample was quantified
`y freeze-drying and distillation to constant specific activity. The total level of
`ability in the rats ranged from 0.3% to 5.9% of administered dose and the majority
`of this labile material remained in the carcass after 48 hr, suggesting that it was
`3H20. It was concluded from these experiments that the low lability would not be
`expected to greatly affect the general metabolic picture but could make a significant
`
`after dosing. Therefore,
`.
`dried routinely prior to analysis by combus-
`
`Absorption
`
`METABOLISM
`
`115
`
`n with the results of intraarterial
`
`Most of the studies reported here were carried out with [3H]buprenorphine as part of
`e drug development program. Another group [Pontani et al., 1985] has also
`eponed ADME studies with the same radiolabel in the rat.
`' The absorption of buprenorphine has been studied in rat, dog, rhesus monkey
`*Brewster et al., 1981a; Numata et al., 1981], rabbit, cynomolgus monkey, and
`'baboon [Lloyd-Jones et al., 1980]. Following intramuscular administration of
`Is of radioactivity peaked at 10—15 min after dosing
`all species (Table I), whereas the absorption peak was delayed following oral
`except the rat), sublingual, and buccal administration of the drug. In general, peak
`.blood levels of buprenorphine were higher after intramuscular doses than after
`arger oral doses owing to extensive first-pass metabolism.
`[In the rat, in vivo studies using in situ isolated intestinal loops and portal vein
`annulation [Castle et al., 1985] showed that buprenorphine administered into the
`loop was extensively metabolized to a conjugate by rat intestine, and all the ab-
`sOrbed drug material following a lO-ug bolus, and 90% following a IOO-ug bolus,
`ppeared as a glucuronide conjugate [Rance and Shillingford, 1977]. The extensive
`t-pass metabolism was accompanied by marked enterohepatic cycling of
`uprenorphine following biliary excretiOn of conjugated buprenorphine and its
`"probable hydrolysis in the lower gut [Brewster et al., 1981a]. Another study in the
`'
`'
`., l98lb] presented the absorption
`of buprenorphine following intra—
`and intraduodenal administration
`
`Page 5
`
`

`

`TABLE 1. Peak Plasma Concentrations of Buprenorphine After Administration
`of [3H]Buprenorphine to Various Species
`
`Species
`Rat
`
`'
`
`1M
`[M
`P0
`1V
`
`1M
`P0
`
`2
`5,000
`40
`5,000
`
`2
`15
`
`~ 1 5
`30
`120
`4
`
`~15
`60
`
`0.8
`805
`0 ,4
`2,290
`
`0.7
`0. 8—3“
`
`Baboon
`
`Rhesus monkey
`
`Cynomolgus monkey
`
`116
`WALTER AND lNTUFlRISl
`
`
`
`
`.‘ger‘..”
`
`.4"?'1'“'r‘1
`
`
`Cm“ (nglg)
`Tmax (min)
`DOSe (pg/kg)
`Route
`fl;
`
`
`4
`10—15
`20
`1M
`I,
`l:-
`370
`15
`5,000
`[M
`E
`
`
`5a
`10
`100
`PO
`.
`'
`PO
`20,000
`‘
`10
`66
`
`SL
`20
`30
`0.4
`
`SL
`200
`60
`1.1
`
`
`
`
`Rabbit
`1M
`5 .000
`15
`132
`
`
`Dog
`1M
`20
`~ 15
`3a
`PO
`100
`60— 1 20
`2—3
`
`PO
`800
`30—60
`10—14
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`1M
`SL
`BU
`
`38
`38
`38
`
`15
`120
`120
`
`4a
`4a
`
`/ a
`
`Value based on total radioactivity.
`
`
`administration. The results show that if the contribution of the intestine to the
`
`metabolism of buprenorphine is bypassed by intrahepatoportal administration, there
`
`is a marked increase in the bioavailability of the drug. Sublingual administration, in
`
`the anaesthetized rat, gave a slower absorption profile than other routes, and the
`
`bioavailability shown in Table 11 is an underestimate of the true value. Results
`
`presented later show that the sublingual route is a satisfactory noninvasive route of
`
`administration of buprenorphine in humans when the bioavailability is of the order
`
`of 55% at normal analgesic doses.
`
`The oral bioavailability of buprenorphine was also low in the dog (mean 7.4%;
`
`range 1.2%—19.7%) after an oral dose of 1 mg/kg. The low oral bioavailability in
`
`animals contrasts with the good bioavailability following intramuscular and subcu—
`taneous injection of the drug. In the baboon the mean intramuscular bioavailability
`
`
`of buprenorphine relative to intravenous administration was 70% [Lloyd-Jones et
`
`al., 1980]. A similar level of intramuscular bioavailability was also obtained in
`
`humans (see below).
`
`For a centrally acting drug like buprenorphine, systemic availability is a neces-
`
`sary prelude to availability to brain tissue. The latter has been studied in rat and
`
`monkey by measuring brain levels of buprenorphine following intramuscular, oral,
`
`
`
`Page 6
`
`Page 6
`
`

`

`METABOLISM
`
`117
`
`U0
`
`9I
`on
`
`E'0
`
`Component in rat brain that had a decay half-life of around 69 hr, an observation that
`
`and sublingual administration. Higher buprenorphine levels were found in brain
`’j following administration by the intramuscular and sublingual routes than by the oral
`' route (Table III). Pontani et a1. [1985] carried out a detailed study of the disposition
`0f buprenorphine in the rat and also showed that buprenorphine was readily avail-
`able to brain tissue following an intravenous dose of 0.2 mg/kg. After 15 min the
`mean brain level of buprenorphine was 117 i 12 ng/g compared with a level of 46
`. i 15 nglml in plasma (Fig. 3). The high brain—to-plasma ratio (Table IV) shows that
`in the rat buprenorphine readily crosses the blood—brain barrier to exert a central
`aetion. The distribution within the brain is predominantly to the cerebrum [Manara
`et al., 1978].
`Pontani et a1. [1985] also described a high-affinity tightly bound buprenorphine
`
`oo1
`
`5
`.E
`1:-
`
`§U
`U
`.E
`
`Or
`
`:UI-
`D.2
`in
`
`Time (h)
`
`Fig. 2. Blood concentrations of buprenorphine in female rats following administration (200
`nglkg) by various routes. (0) Intraarterial; (A) intravenous; (O) rectal; (A) intrahepatopor—
`tal; (El) sublingual; (I) intraduodenal. Points represent mean values i SEM of four animals.
`Reproduced from Brewster et al. [1981b], with permission of the publisher.
`
`Page 7
`
`

`

`WALTER AND INTURFllSI
`
`TABLE II. Relative Bioavailabilities of Buprenorphine in the Rat
`for Various Routes Over the Period 0—4 hr After Dosing”
`__—______’____———————
`Area under blood
`Relative systemic
`concentration time curve
`availability (%) over
`Route
`(AUCMM, ng/ ml/ min)
`the period 0—4 hr°
`f,—
`
`1,852 ‘i 189
`lntraarterial
`1,807 i 242
`Intravenous
`1,000
`267
`lntrarectal
`900
`161
`lntrahepatoportal
`2
`249 _ 39
`Sublinguald
`\llfi
`9.
`4
`180 i 71
`[ntraduodenal
`___________—__—-——————
`
`100
`98
`
`33% 1+1+1+1+1+
`
`13
`14
`9
`
`“Value based on total radioactivity.
`
`TABLE III. Peak Brain Concentrations of Buprenorphine After Administration of
`[3H]Buprenorphine to Rat and Rhesus Monkey
`_______________’——————
`Species
`Route
`Dose (pg/kg)
`Tm” (min)
`Cm“ (ng/g)
`___—_____P_p_————————-
`Rat
`1M
`20
`40
`6“
`IM
`20
`6O
`14
`P0
`80
`2“
`P0
`51..
`
`“Reproduced from Brewster et al. [1981b], with permission of the publisher.
`1)Values are the mean for four animals : SEM.
`L'lntrztarterial route assigned to represent complete availability.
`dThe slow absorption profile for this route results in a considerable underestimate
`of the sublingual availability.
`
`they believe is consistent with the known high-affinity, slow-dissociation binding to
`opiate receptors.
`
`Distribution
`
`.
`
`Tissue distribution studies have been carried out in the rat. Pontani et al. [1985]
`studied the distribution of a ZOO-ug/kg intravenous dose of [3H]buprenorphine
`(Table IV) and found a distribution similar to that in another study, carried out as
`part of the development program, following a 20-p.g/kg intramuscular dose (Table
`V). At early times after dosing, high tissue levels were found in lung, heart, kidney,
`and liver. Brain levels were higher than plasma levels in both studies, and separate
`studies showed that most of the brain radioactivity was unchanged drug. A study of
`
`20
`
`5
`
`4°
`60
`2
`1M
`Rhesus monkey
`0. 3°
`60
`15
`P0
`_’_______________———————-
`
`Page 8
`
`Page 8
`
`

`

`METABOLISM
`
`119
`
`Bound drug
`'(brain)
`
`Bound drug
`(brain)
`
`010
`
`HNOO
`
`01
`
`N
`
`
`
`
`
`(nglgtissueormlflutd)
`
`
`
`Buprenorphineconcentration
`
`
`
`
`
`new.‘:
`
`the distribution of [3H]buprenorphine in rats following a 20—mg/kg oral dose
`showed that maximum tissue radioactivity levels generally occurred within 1 hr of
`dosage. The highest levels were found in the excretory organs, liver and kidney.
`Again,
`the radioactivity in brain consisted almost entirely of unchanged drug,
`showing that such drug as survived first-pass metabolism was able to cross the
`blood—brain barrier. Chromatography studies showed that after intramuscular dos—
`ing of 20 ug/kg only 10%—36% of the radioactivity in liver was unchanged
`buprenorphine, whereas in other tissues, up to 30 min after dosing, most of the
`radiolabel (70%—97%) was the unchanged drug.
`The distribution of buprenorphine was also compared in pregnant and nonpreg-
`nant rats, as were the levels of drug and metabolites measured in 11- and 21—day
`fetal tissues following a 5—mg/kg intramuscular close of [3H] buprenorphine. Levels
`of radioactivity in plasma and selected tissues were similar in pregnant and nonpreg-
`nant female rats, and the proportions of free dmg in plasma were also similar,
`indicating that they had a similar metabolic capacity. Radioactivity was present in
`
`(Plasma)
`
`46
`
`481624324048
`
`Time after injection (H)
`
`Fig. 3. Decay of extractable buprenorphine from brain and plasma and of bound drug in
`brain of rats injected with a single intravenous bolus dose of [3H] buprenorphine (0.2 mg/kg).
`Data represent means i SEM from three animals at each time up to 6 hr (a). and for times
`later than 6 hr (b). Reproduced from Pontani et al. [1985], with permission of the publisher.
`
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`Page 10
`
`Page 10
`
`
`
`
`

`

`METABOLISM
`
`121
`
`the fetus at both 11 and 21 days, showing that drug-related material passed the
`placental barrier. In the 21-day rat fetus most of the radioactivity in the fetal
`gastrointestinal
`tract was
`as polar conjugates. Other
`studies
`showed that
`buprenorphine injected subcutaneously into female rats at the same high dose of 5
`mg/kg/day for 14 days prior to mating, during mating, and throughout the gestation
`period caused no effect on fertility or gestation indices [Sutton et a1., 1986].
`
`Metabolism
`
`Metabolite identification was carried out on the excretion products from rat, rabbit,
`dog, baboon, and rhesus monkey and the metabolic pattern was found to be similar
`in all species studied. Two pathways—conjugation with glucuronic acid and
`N—dealkyla’tion—are well documented, leading to at least three metabolites: bupre-
`norphine conjugate, N-dealkyl buprenorphine, and N-dealkyl buprenorphine conju-
`gate (Fig. 4). In the rat and dog other nonhydrolyzable polar metabolites have been
`
`TABLE V. Distribution of Radioactivity in 'lissues of Male Sprague-Dawley Rats Injected
`Intramuscularly With a Single Bolus Dose of [3H]Buprenorphine (0.02 mg/kg)n
`if
`Time after
`Distribution of radioactiVity (ng/g equivalent)
`
`0.25 . 0.5 1.0 2.0 4.0 6.0injection (hr) 24/
`
`
`
`
`
`
`Plasma
`3.0
`2.1
`1.3
`0.4
`0.6
`5.5
`0.1
`Brain
`9.4
`11.9
`8.7
`6.6
`5.3
`3.5
`0.4
`Liver
`25.8
`24.5
`17.8
`13.7
`8.2
`9.8
`3.4
`Heart
`21.1
`12.7
`6.4
`4.3
`1.6
`1.6
`0.3
`Lung
`34.6
`21.4
`11.0
`7.8
`3.4
`3.5
`0.8
`Kidney
`29.3
`19.4
`10.6
`7.7
`3.3
`4.2
`2.3
`Spleen
`5.7
`12.3
`7.1
`4.1
`2.9
`3.3
`0.9
`Testes
`2.7
`3.8
`4.0
`3.1
`0.8
`0.7
`0.1
`Muscle (diaphragm)
`15.3
`10.4
`5.7
`3.9
`1.3
`1.2
`0.2
`
`12.3 24.5 28.9 29.5 33.0 15.7Fat 1.6f
`
`
`
`
`
`
`“Data represent mean from two male animals at each time. The distribution in female rats carried out at
`the same time was similar.
`
`u.‘u.WflrfiUh€¥l"¢k\.-A-~.»
`
`Buprenorphine
`
`Buprenorphine conjugate
`
`N-dealkyl buprenorphine
`I
`I.
`I
`
`N—dealkyl buprenorphine
`conjugate
`
`.
`6-O—desmethyl—
`6—O—desmethy1-
`N-dealkyl buprenorphine--- N—dealkyl buprenorphlne
`conjugate
`
`, Known animal
`Fig. 4. Known and possible metabolic pathways for buprenorphine.
`and human metabolic pathways; —————, metabolic pathway in Wistar rats.
`
`Page 11
`
`

`

`1 22
`
`WALTER AND INTUHRISI
`
`TABLE VI. Proportion of Radioactivity Attributable
`to Buprenorphine and Its Metabolites in Rat Feces
`Following 4.5 mg/kg i.v., 4.5 mg/kg i.m.,
`and 80 mg/kg p.o. of [3H]Buprenorphine
`—_——————
`Percentage of radioactivity
`
`i.v.
`
`p.o.
`
`i.m.
`
`.
`
`l
`I
`{l
`|
`
`micr
`sho
`mo ‘
`
`Ex
`A54
`1y
`con
`cre‘
`int
`Obt
`SU
`1;“0
`
`
`
`
`
`
`
`
`
`
`
`Buprenorphine
`34
`61
`36
`
`
`N—Dealkyl buprenorphine
`63
`23
`43
`
`Nonhydrolyzable polar metabolites
`4
`15
`20
`
`
`y (Tables VI and VII). One of these may be
`observed by thin-layer chromatograph
`6-O-desmethyl norbuprenorphine, a polar metabolite tentatively identified by Pon-
`tani et a1.
`[1985] in Wistar rat urine; a conjugate of this metabolite was also
`
`
`observed (Table VIII).
`The proportions of buprenorphine and its metabolites in rat and dog feces after
`
`
`intravenous, oral , and intramuscular administration are shown in Tables VI and VII.
`
`
`In rat, more N-dealkylation occurred following intravenous and intramuscular dos—
`ing and, conversely, more unchanged drug appeared in feces after oral dosing.
`There was also a higher proportion of the unknown polar metabolites in rat feces
`
`
`
`
`following oral and intramuscular dosing compared with intravenous dosing. In
`contrast,
`the metabolite patterns in dog feces after intravenous, oral, and intra—
`
`
`muscular dosing were very similar (Table VII).
`
`
`Rat bile samples contained over 94% of polar metabolites (Table IX). After
`
`
`enzyme hydrolysis around 70% of the radioactivity was hydrolyzed to buprenor-
`
`
`phine and N-dealkyl buprenorphine, but around 20% was made up of nonhydrolyz-
`able metabolites. There was a sex difference in the amount of N—dealkylation that
`
`
`
`
`
`
`198121].
`was apparently greater in male rats [Brewster et al.,
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`'m
`iv
`l
`I
`.. ./p.0. 1.
`
`
`
`
`
`81
`93
`80
`Buprenorphine
`
`
`N-Dealkyl buprenorphine
`4
`6
`4
`
`
`14
`l 1
`3
`Nonhydrolyzable polar metabolites/
`
`
`Effects on Drug-Metabolizing Enzymes
`
`muscularly with buprenorphine (0.1 or 4
`Male and female rats were closed intra
`y for 4 days to study the effects on hepatic
`mg/kg) or morphine (3 mg/kg) twice dail
`
`TABLE VII. Proportion of Radioactivity Attributable
`to Buprenorphine and Its Metabolites in Dog Feces
`Following 1.5 mglkg i.v., 1.5 mg/kg i.m.,
`and 15 mg/kg p.o. of [3H]Buprenorphine
`Percentage of radioactivity/
`
`l
`
`.
`
`
`
`Page 12
`
`Page 12
`
`

`

`METABOLISM
`
`123
`
`TABLE VIII. Proportions of Total Radioactivity as Metabolites
`in Unhydrolyzed Rat Urine (l-Week Sample) Following Dosing
`With 0.2 mg/kg i.v. of [3H]Buprenorphinea
`
`Compound
`
`%
`
`1 .9
`Buprenorphine
`0.5—0.9
`Buprenorphine conjugate
`9.4
`N-Dealkyl buprenorphine
`5.2
`N-Dealkyl buprenorphine conjugate
`5.4
`6—O-Desmethyl N-dealkyl buprenorphine
`
`6-O—Desmethyl N-dealkyl buprenorphine conjugate 15.9
`IIReproduced from Pontani et al. [1985], with permission of the publisher.
`
`bValues represent means 1 SEM of three animals.
`
`As previously discussed, the elimination of buprenorphine—related material is main—
`.ly via the feces following biliary excretion of conjugated unchanged drug and
`conjugated phase I metabolite(s). A small amount of drug—related material is ex-
`creted in the urine. Balance study data from rat, dog, and rhesus monkey following
`intramuscular administration are presented in Table X. Similar results have been
`obtained following intravenous and oral administration in rat and dog and following
`sublingual administration to rats: [n the latter a mean 79.6% of a 0.1—mg/kg sub-
`lingual dose of [3H]buprenorphine was excreted in feces and a mean of 3.6% of
`' dose in urine (three rats) over a 48-hr collection period.
`
`microsomal enzyme activity. There was a difference between the sexes; male rats
`showed an increase in microsomal enzyme activity following buprenorphine and
`Vimorphine, whereas the activity in female rats was unaffected.
`
`Excretion
`
`TABLE IX.’ Proportion of Bile Radioactivity
`(0- to 24-hr Sample) in Male and Female Rats
`Following 100 [Lg/kg i.v. of [3H]Buprenorphineavb
`Male
`
`Before enzyme hydrolysis
`Buprenorphine
`N-Dealkyl buprenorphine
`Conjugates + nonhydrolyzable
`polar metabolites
`
`After enzyme hydrolysis
`'
`Buprenorphine
`N-Dealkyl buprenorphine
`Nonhydrolyzable polar metabolites
`
`1.5 i 0.8
`0
`94.2 i 1.0
`
`55.2 i 2.2
`15.3 i 1.8
`23.0 i 2.5
`
`72.5 i 0.4
`1.4 i- 0.7
`18.9 t 1.0
`
`x‘Reproduced from Brewster et al. [1981a], with permission of the publisher.
`
`Page 13
`
`

`

`124
`
`WALTER AND lNTUFiHISl
`
`TABLE X. Cumulative Excretion of Radioactivity
`by Various Species Following a Single Intramuscular
`Dose of [3H]Buprenorphine“,h
`
`Collection period (hr)
`
`Species
`Sample
`0—24
`0—72
`0— 144
`________f__————————
`Rat
`Urine
`5.1
`6.9
`7.1
`Feces
`19.9
`82.6
`83.6
`Total
`25.0
`89.5
`90.7“
`
`Dog
`
`Urine
`Feces
`Total
`
`3.0
`0.0
`3.0
`
`5.1
`85.6
`90.7
`
`5.8
`94.5
`100.3
`
`Rhesus monkey
`
`18.5
`12.7
`7.4
`Urine
`64.1
`52.9
`22.6
`Feces
`82.6
`65.6
`30.0
`Total
`__’___’———————
`
`"Reproduced from Brewster et al. [1981a], with permission of the publisher.
`bValues given for each animal species represent the mean result for two rats
`(female), two dogs (1 male and 1 female), and two rhesus monkeys (female) at
`doses of 20, 20, and 2 rig/kg, respectively.
`L‘Collection period 0—96 hr only.
`
`
`
`“Reproduced from Brewster et al. [1981a], with permission of the publisher.
`
`In all species it is likely that there is substantial enterohepatic cycling of drug and
`metabolites that gives rise to a slow excretion rate. In the rat, Brewster et al. [1981a]
`showed that the proportion of N—dealkyl buprenorphine in the bile was increased
`with each enterohepatic cycle (Table XI),
`indicating that the slow excretion of
`buprenorphine is linked with further biotransformation. The significance of these
`observations in the interpretation of human data is discussed later.
`
`Excretion of Buprenorphine in the Milk of Lactating Female Rats
`
`The transfer of buprenorphine and its metabolites to neonates via milk has been
`studied in lactating female rats following a single intramuscular injection of
`
`TABLE Xl. Change in Proportion of Metabolites
`Following Enterohepatic Cycling of Radiolabeled
`Material in Two Male Ratsx1
`_—__—___—___—————-
`% of bile radioactivity
`(hydrolyzed samples)
`
`2nd cycle
`lst cycle
`__4____—_—__———————
`
`36.8
`54.4
`Buprenorphine
`46.1
`25.0
`N-Dealkyl buprenorphine
`
`
`17.1Nonhydrolyzable polar metabolites 16.8f!
`
`Page 14
`
`Page 14
`
`

`

`METABOLISM
`
`125
`
`TABLE XII. Concentrations of Free Buprenorphine and Metabolites (ug or ug
`equiv. per 1 ml fluid or 1 g tissue) in Male Wistar Rats Each Implanted With a
`[15,16(n)-[3H]Buprenorphine Pellet for Various Times"-b/
`Time (weeks)
`
`
`
`
`
`
`
`
`4 6 102 12/
`Plasma
`0.006 (——)
`0.098 (29)
`0.044 (——)
`0.007 (16.7)
`0.018 (9.7)
`Liver
`0.043 (8.8)
`0.055 (5.1)
`0.017 (6.2)
`0.020 (4.0)
`0.016 (4.3)
`Heart
`0.065 (6.5)
`0.041 (4.2)
`0.063 (2.6)
`0.063 (1.4)
`0.050 (1.3)
`Lung
`0.054 (5.4)
`0.031 (0.7)
`0.045 (0.6)
`0.013 (1.1)
`0.014 (0.6)
`Kidney
`0.070 (4.8)
`0.042 (2.3)
`0.057 (1.9)
`0.016 (0.9)
`0.022 (0.1)
`Spleen
`0.208 (7.7)
`0.096 (6.7)
`0.137 (4.5)
`0.055 (2.4)
`0.048 (0.7)
`Testes
`0.016 (6.5)
`0.008 (4.2)
`0.008 (2.4)
`0.010 (2.7)
`0.008 (1.7)
`Skeletal
`0.350 (11.1)
`0.204 (7.5)
`0.362 (6.3)
`0.067 (3.5)
`0.109 (2.4)
`muscle
`Fat
`
`Colman drug development program. The human pharmacokinetics of buprenor~
`
`0.175 (33.7)
`
`0.187 (33.8)
`
`0.140 (21.4)
`
`0.261 (19.3)
`
`0.366 (26.7)
`
`3Reproduced from Pontani et a1. [1985], with permission of the publisher.
`hData represent mean values from two animals. The concentrations of metabolites are given in parenthe-
`ses. Plasma samples at 2 and 6 weeks were lost during determination of metabolite concentrations. Total
`radioactivity values were obtained by combustion of aliquots of tissue homogenates in a biological tissue
`oxidizer, and metabolite concentrations were determined by subtraction of the concentration of free
`buprenorphine from total radioactivity values. The dose of [3H] Buprenorphine was 10 mg.
`
`[3H]buprenorphine (5 mg/kg). The studies showed that drug-related material is
`excreted in the milk of rats and that concentrations of unchanged buprenorphine in
`milk can equal or exceed that in plasma. From these data it seems likely that
`buprenorphine would be excreted in human breast milk.
`
`Chronic Dosing of Buprenorphine
`Pontani et al. [1985] studied the distribution of [3H]buprenorphine in the rat follow-
`ing its continuous release from a subcutaneously implanted pellet. The dose con—
`tained in the pellet was 10 mg of buprenorphine and the release was monitored over
`12 weeks. The levels of unchanged buprenorphine and total metabolites in a number
`of tissues are given in Table XII. The results from chronic dosing show that metabo-
`lites form the major radioactive component of rat plasma, as would be expected
`because of the slow elimination of drug metabolites owing to enterohepatic circula-
`tion.
`
`ADME STUDIES IN HUMANS.
`
`The absorption, plasma levels, metabolism, and excretion of buprenorphine in
`humans have been examined in a number of studies, some as part of the Reckitt &
`
`Page 15
`
`

`

`
`
`
`
`
`
`' "Wfitfiém‘?
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`'
`
`
`
`
`
`
`
`
`126
`
`WALTER AND INTUHHISI
`
`
`phine are discussed by McQuay and Moore in this volume and are briefly consid-
`ered here for comparison with animal results.
`Four routes of administration have been examined: intravenous, intramuscular,
`sublingual, and oral. A major problem with all the pharmacokinetic studies has been
`the lack of a robust, reproducible assay that is specific for buprenorphine (partic-
`ularly against the N—dealkyl metabolite) and retains sufficient sensitivity to measure
`low levels of drug in plasma especially at late times after dosing. HPIE methods
`were able to provide the specificity but none of the detection systems provided the
`required sensitivity. GC/MS methods provided both the sensitivity and specificity
`but were not capable of handling a large throughput of samples. The GC/MS
`method of Blom et al. [1985] served to describe the elimination of buprenorphine
`after intravenous administration of the drug and to measure the N—dealkyl metabolite
`for a few hours after dosing. However, application of this method for routine
`analysis was unsuccessful. Similar results with GC/MS were obtained more recent-
`ly by Ohtani et a1. [1989], who measured buprenorphine and N-dealkyl buprenor-
`phine plasma profiles in one volunteer after a sublingual dose of the drug (Fig. 5).
`Again, the N-dealkyl metabolite was measurable only between two and three hours
`after dosing. There is no information about the robustness of 'this new method in
`routine analysis.
`In most of the single-dose absorption studies, human plasma buprenorphine
`levels have been obtained by the best available assay for the drug, which is the
`radioimmunoassay method developed by Bartlett et al. [1980]. In early studies,
`[3H]buprenorphine was used as the radioligand but this method was later modified
`to allow the use of [’35[]—labeled buprenorphine [Hand et a1., 1986]. Although this
`assay does not distinguish between buprenorphine and N-dealkyl buprenorphine,
`the results of Blom et a1. [1985] and Ohtani et al. [1989] have shown that this
`metabolite is measurable only at early times after dosing. However, it is likely that
`after a single dose the plasma immunoreactivity profiles will be made up in part
`from N~dealkyl buprenorphine and mostly from buprenorphine.
`
`
`
`ionof
`
`—|
`
`
`
`PlasmaconcentratBNandNBN(ng/ml)
`
`
`3
`
`789
`6
`5
`4
`2
`
`
`Time (h)
`
`Fig. 5. Time courses of plasma concentrations of buprenorphine (BN) and nor-buprenorphine
`(NBN) after administration of two sublingual tablets of buprenorphine to a healthy volunteer.
`(O) Buprenorphine; (I) norbuprenorphine. Adapted from Ohtani et al. [1989].
`
`
`
`Page 16
`
`Page 16
`
`

`

`METABOLISM
`
`127
`
`In chronic dosing studies, because of the slow excretion of drug-related material
`that is due to enterohepatic circulation of buprenorphine and metabolites, the simple
`- radioimmunoassay method will not provide a useful picture of the levels of
`' buprenorphine in view of the high levels of metabolites. A more suitable method has
`been described by Hand et al. [1986] in which immunoreactivity levels of diethyl
`ether-extracted buprenorphine are subtracted from total immunoreactivity levels to
`provide information about the levels of N-dealkyl buprenorphine and buprenorphine
`. glucuronide, the latter two being distinguished from each other by using the two
`‘, antibodies with selective metabolite cross~reactivity.
`‘
`Early excretion balance studies were carried out with [3H]buprenorphine. A later
`study by Cone et al. [1984] examined the excretion and metabolism of high sub-
`lingual doses of buprenorphine by applying GC/MS techniques to analyze the
`samples.
`
`affirminal elimination rate was observed in the single patient studied by Ohtani et a1.
`
`Single-Dose Absorption Studies
`Early human pharmacology studies showed that oral doses of buprenorphine at least
`. 0 times higher than intramuscular doses were needed to exert equivalent phar—
`"macological effects. Excretion studies following an oral dose of 20 pug/kg of
`[3H]buprenorphine showed that radioactivity was absorbed but the identity of the
`radioact