No. . 2
`
`'
`
`29(2) 5].9——5E1 (1961)
`[Chem. Pharm. Bull.)
`
`519
`
`Studies on the Absorption of practically Water—insoluble Drugs following
`Injection.
`1.
`Intramuscular Absorption from Water-
`immiscible Oil Solutions in Rats
`
`Korcmno HiRANo,* TERUHISA ICHIHASHI, and Hinro YAMADA
`
`Sh-ionogi Research Laboratories, Shionogi and C0,, Ltd, Sogzlm 542-4, ,
`Fukushime-ku,. Osaka, 533, japan
`
`(Received Iuly B0, 1980)
`
`The absorption behavior following intramuscular injection of practically water-
`lnsoluble drugs in Water-immiscible oil solution was investigated with several azo dyes
`and two steroids as model drugs by the local clearance method in the m. gaslrocmemius
`of the rat. The absorption oi the drug component obeyed approximately 1st order
`kinetics, while the absorption of the oily solvent was very ‘slow. The injection volume
`(V0) influenced the absorption rate constant (k) and the correlation .7: cc V0“ was experi-
`mentally observed (ior intact rats, m=—0.14; ior anesthetized rats, m:#0.32). This
`was also the case at another injection site,
`the m. reams femoris in rats. Comparison
`of the absorption rate of a drug in various oil vehicles showed that I: was controlled predo-
`minantly by the oilfwater distribution coefficient (K) and depended little on the viscosity
`of the vehicle. These results suggested that the release process of the drug component
`from the oily depot to the aqueous phase around it was the main route for absorption,
`and that the subsequent transport process in the aqueous phase might be ratevlirniting.
`A plot of log is versus log K gave a straight line with a slope close to -1, but of a slightly
`Smaller absolute value. This piot could be applied satisfactorily for estimating the
`absorption rate of other drug-oil systems. A guideline for predicting the absorption rate
`of a drug in oily suspension irom the I: value for its solution is presented.
`Keywords-——-drug absorption kinetics;
`intramuscular injection;
`local clearance
`method;
`practically water—insoluble drugs; waterdmmiscible oil vehicles;
`injection
`volume; distribution Coefficient;
`intact and anesthetized rats
`
`Parenteral routes of drug administration are as useful and important as oral ones for early
`screening and preclinical testing of drugs in animals. There are many kinds of injections,
`including intramuscular, subcutaneous, intravenous, intradermal, hypoclermal, intraarterial,
`intrapleural, intraperitoneal, intraarticular, intracardial, intraspinal anclintracerebral. Ab«
`sorption is not involved when a drug is administered parenterally by an intravasular route.
`However, when the drug is administered by any extravascular route, a depot of some type
`is formed and the drug must leave the depot and reach the blood or lymph systems by some
`process(es).
`Since the publication of several reviews“ there has been increasing interest in the absorp-
`tion behavior of drugs in aqueous solution from intramuscular“ and subcutaneous sites.”
`However, compared to Watensoluble drugs, little work has been done on practically water-
`insoluble drugs. These drugs are commonly administered to laboratory animals as parenteral
`preparations in the form of oily solutions (oily suspensions), aqueous suspensions, aqueous
`solutions soluhilized with nonionic surfactants, or emulsions. The absorption of such drugs,
`which depends on the dosage form or formulation,
`is in most cases slow enough to be rate-
`determinant in their disposition in the body. This means that even pharmacological responses
`are sometimes governed primarily by the formulation itself. Therefore, basic investigation
`on drug absorption from such dosage forms by each parenteral route is required in order to
`select the optimal preparation and parenteral route for more complete screening tests in ani-
`mals.
`
`The present study was undertaken to elucidate the absorption behavior of practically
`water-insoluble drugs from Water—immiscible oil solutions injected intramuscularly. Early
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`520 Vol. 29 (1931)
`
`investigators“ noted mainly the vehicle effect on the pharmacologic responses or blood levels
`of drugs but did not refer to the mechanism or kinetics of drug absorption from such prepara-
`tions. Recently, Tanaka et calf” studied these problems using slightly water—soiuble drugs
`in anesthetized and operated rats.
`For our study, several azo dyes and some steroids which are unionized and practically
`water-insoluble under physiological conditions were used as model drugs, and their intramus-
`cular absorption properties were examined by the local clearance method in the m. gastro-
`cnemius of the intact rat. Under various experimental systems and conditions, the intramu-
`scular absorption rates of these model drugs were compared to evaluate the contribution of
`. physicochemical iactors and to clarify the kinetic process. On the basis of the results, predic-
`tions of the absorption rates of other drugs in oily solutions were attempted. The relationship
`between absorption rates in oily solution and oily suspension was also investigated.
`
`Experirnen tal
`
`Azo dyes such as ;b—aminoazobenzene (PAAB), gwhydroxyazobenzene (PHAB), 0-amino-
`Materials
`azotoluene (OAAT), 1—phenylazo—2-naphthylamine (PANA) and tetrazobenzeneqflinaphthol
`(Sudan III),
`and steroids such as testosterone (TS) and 2cc,3as-epithio—5ac-a.ndrostan~17,8~ol (epitiostanol) were selected as
`model compounds for practically water-insoluble drugs, These compounds (other than epitiostanol) were
`obtained commercially: PAAB, PANA and TS, Tokyo Kasei Kogyo Co., Ltd. (Tokyo); PI-IAB, Eastman
`Kodak Co. (N.Y.); OAAT, Ishizu Pharmaceutical Co., Ltd. (Osalra); Sudan III, E. Merck AG (Darmstadt).
`These compounds were of reagent grade and were used without further purification, except for PHAB,
`which was purified by being dissolved in MeOH then recrystallized with water. Epitiostanol was synthesized
`in our laboratory and was of medicinal grade. As watenimmiscible oil solvents for the injection preparations,
`sesame oil (SO), medium chain (C,,—C1,) triglyceride commercially named Miglyol 812 (Mig), isopropyl myria-
`tate (IPM), diethyl sebacate (DES) and castor oil (CO) were selected in view of relatively wide variety in
`viscosity and dissolving powder, These compounds Were also obtained from commercial sources: SO,
`Maruishi Pharmaceutical Co., Ltd. (Osaka); Mig, Chemische Werke Witten (Germany); IPM and DES,
`Nikko Chemicals Co., Ltd. (Tokyo); CO, Kenei Pharmaceutical Co. (Japan).
`SO and CO were of JP grade
`and the others were of reagent grade. All were used without further purification. All other chemicals
`used in this investigation were of analytical or reagent grade.
`Test Injection Preparations
`An oily solution was prepared by dissolving the desired amount of a
`test compound in an oily solvent and filtering it through an SM 116 membrane filter (Sartorius-Membranfilter
`Grnbl-I, Gottingen). All test solutions used here were ascertained to be chemically and physically stable
`for at least the experimental period. An oily suspension of PAAB or TS was formulated in a room held at 37°
`by dispersing the sieved powder (for PAAB, 74——149 pm in diameter; for TS, 37~74 and 74-149 (mi) in the
`oil vehicle presaturatcd with each drug using a mixer (Micro Thermo Mixer Model TM-101, Thermonics
`C0,, Ltd., Tokyo), and the product was stored at 37“ until use. All suspensions were used about 20 hr after
`preparation.
`Animal Experimcn ts———-Male Wistar albino rats weighing 250: 30 g were used in all animal experiments.
`(i) Absorption Experiment Procedure: The injection site for the present study, unless otherwise
`mentioned, was the medial head of the m. gastromemius in the m. triceps sums of the left hind leg in the
`rat; this site was selected because the location of the needle tip could be easily controlled. The absorption
`time course was followed by the local clearance method. The rat was given light anesthesia with ether,
`then a thin Terumo needle (27G X 3,'4” for oily solutions and 25G X 1'’ for oily suspensions; Terumo Co., Ltd.,
`Tokyo) connected to a volume sca1e~corrected syringe or Terumo micrometer syringe (MS 10, 50 and 100)
`was inserted a few millimeters above the ankle and the tip of the needle was led to the center of the m. gastro-
`memius, medial head. Next, the test solution (50 (1.1, unless otherwise stated) was injected at moderate
`speed.
`Immediately after withdrawal of the needle, the quick adhesive Aron Alpha (Toa Gousei Kagaku
`Kogyo Co., Ltd., Tokyo) was applied to the insertion site in order to prevent leakage of the oily solution
`injected. During the absorption experiment, the rat was housed in a cage which allowed free movement
`and easy access to water and food. At various intervals after the injection, rats were decapitated and bled,
`and the muscle tissues around the injection site, including the oily depot, were excised as completely as
`possible. The removed muscles were mixed with 5 ml of water and homogenized with a high speed blender
`(Ultra-Turrax, Ianke and Kunlrel K.G., Ger.) under cooling in an ice-water bath, Another 5 ml of water
`was used to wash the blender of the homogenizer and this was added to the above homogenate. After
`extraction from the homogenate with 8 ml of ethyl acetate, the residual amount of the drug in the removed
`muscles was analyzed. For absorption studies with sesame oil (30), the following indirect method was
`adopted. Fifty microliters of 80 solution containing Sudan III (5 mgfml) was administered to the same
`site in the manner described above in two groups of rats.
`= At a set time interval, both groups were sacrificed.
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`From one group, the residual amount (W) of Sudan III was determined, and from the other, its concentration
`(C) in S0 solution remaining in the muscle was determined. Since Sudan III was pooled and kept homo-
`geneously dissolved in the SO depot for at least 20 hr after injection, the volume (V) of 80 remaining in the
`muscle could be approximateiy calculated as I/'=W[C.
`(ii)
`In Vitro Incubation Experiment:
`In order to ascertain whether the clearance of a given‘ drug
`from the injection site couid be attributed to its transport into the vascular systems or not, an in mitt/o incuba-
`tion experiment was undertaken along with the absorption experiment, and the metabolic change at the
`injection site was checked. Fifty microliters of 80 solution containing 0.5 mgfml of an azo compound was
`injected into the same site in the manner described above in two groups of rats. For one group, the muscles,
`including the injected solution, were removed immediately after administration, then incubated in 5 ml
`of Ringer-Locke solution under bubbling with air and mild shaking at 37° for 2 or 6 hr (Yamato incubator,
`model BT-41; Yamato Scientific Co., Ltd., Tokyo). After incubation, the residual amount of the test com—
`pound was analyzed and compared with that obtained from the absorption experiment in another group of
`rats. Visible absorption spectra were also measured before and after the above experiments with a Hitachi
`EPS-3T recording spectrophotometer (Hitachi Co., Ltd., Tokyo) to check for metabolic change of the test
`compounds.
`Kinematic viscosities of oily solvents were determined with an
`Measurement of Absolute Viscosity
`Ubbelohde viscometer (Kaburagi Kagakn Kikai Kogyo Co., Ltd., Japan) and their densities were measured
`by using avolume scale-corrected Cassia flask. These determinations were performed at 37“, and the absolute
`viscosity was calculated from the kinematic viscosity and density.
`Determination of the Apparent Distribution Coeflirxient
`Ten milliliters of 0.9% (wfvj Na.Cl aqueous
`solution (saline) was added to 4 1:11 of an oily solution containing the mode}. compound (concentration: for
`PAAB, PHAB and OAAT, 5—4O mgfml; for TS, 2.33—-4.65 mg/ml; for epitiostanol, 2—5 mgfml) and shaken
`well at 37° with a mechanical shaker until distribution equilibrium was reached. Our preliminary experiment
`had demonstrated that 2 hr was needed for this equilibrium, and that the volume change of each phase after
`equilibrium was small enough to neglect. After equilibrium had been reached, the unstable emulsion was
`centrifuged at 4000 rpm for 10 min to separate the transparent aqueous phase, part of which was withdrawn
`fior analysis.
`‘The concentration of the model drug in the oily phase was calculated from its concentration
`determined for the aqueous phase and its initial concentration in the oily phase. The apparent distribution
`coefficient (K) was represented as the concentration ratio between the two phases. The value of K as deter-
`mined above depended little on the concentration and was similar to the‘ solubility ratio at 37° between
`the oil and saline solvents. For PANA, whose concentration in saline was too low to be determined accurate-
`ly, the solubility ratio between the two solvents was used as the apparent distribution coefficient.
`Analytical Method
`(i) Samples from Animal Experiments: Sample ethyl acetate solutions contain-
`ing azo compounds were adequately diluted with ethyl acetate and their optical densities were measured with
`a. Hitachi UV-VIS spectrophotometer at 384-, 348, 386, 450 and 510 nm for PAAB, PHAB, OAAT, PANA
`and Sudan. III, respectively. Epitiostanol was determined by GLC after being transformed to olefin (5ac»
`androst—en—17,B—ol)
`in the following manner.” Five milliliters of sample solution (epitiostanol content,
`0-460 pg) "was shaken with 1 g of A130, (active, neutral, I, E. Merck, Darmstadt) for 10 min and centrifuged
`at 3006 rpm for 5 min (this procedure was done to minimize interfering materials in the following GLC assay).
`To 1.5 ml of the supernatant was added 0.5 ml of internal standard solution (3)3-acetoxy—5aL-androstan in
`ethyl acetate, 0.25 mg/ml), then the mixture was evaporated to dryness in a Vapour Mix instrument (Tokyo
`Rikakikai Co., Ltd., Tokyo). The residue was dissolved in 5 ml of benzene and refluxed with about 30 mg
`of spongy active copper (Cu—Zn) for 40 min. The reaction mixture was filtered through Toyo filter paper
`No. 131, and 2 ml of the filtrate was. evaporated to dryness. The residue was redissolved in about 200 pl
`of ethyl acetate and 1-W2 pl of this was analyzed with a. gas chromatograph, model GLC—4APTF, equipped
`with a. flame ionization detector (Shimadzu Seisaicusho, Ltd., Kyoto) under the following conditions. The
`column was a 1.5 m><5 mm glass tube packed with 3% SE-30 on Gas Chrom Q (80-1(i0 mesh) ; carrier gas,‘
`99.999% N, (flow rate, 60 mllmin); column, injection port and detector temperatures, 250, 268 and 288°,-
`respectively.
`In the case of TS, 3 ml of sample solution was shaken with 0.6 g of an A'1,O3-silica. gel’) mixture
`(1: 1, w/w) for 10 min then centrifuged at 3000 rpm for 5 min (this procedure was performed for the same
`reason as in the case of epitiostauol). To 1 ml of the supernatant was added 100 ul of internal standard
`solution (35-acetoxy-5a-androstan in ethyl acetate,‘ 0.5 mg/ml). Two microliters of this mixture was used
`for GLC analysislas described above (coiumn, injection port and detector temperatures were modified to
`260, 280 and 293°, respectively).
`‘
`.
`_
`(ii) Samples from Other Experiments:
`(1) Test Compounds in Oily Solutions:
`Sample Solutions for
`am compounds were analyzed colorime-trically after dilution with chloroform (PAAB, 372 nm; PHAB, 345
`nm; OAAT, 376 nm; PANA, 436 nm; Sudan III, 520 nm). Epitiostauol and T8 were assayed by GLC as
`described above after dilution with ethyl acetate.
`(2) Test Compounds in Saline: Sample solutions of azo
`compounds and TS were analyzed spectrophotometricaily as follows: PAAB and PHAB at 376 and 348 um,
`respectively, after‘ dilution with distilled water; OAAT and PANA at 379 and 470 nm, respectively, after
`10f9-fold dilution with EtOH; TS at 241 nm after 1G~fold dilution with EtOH. Epitiostanol was assayed
`by the GLC method described abovelafter extraction with ethyl acetate.
`‘
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`Results
`
`Comparison of Clearance of Model Compounds in the Injection Site between in Vitro Incubation
`and in Vivo Absorption Experiments
`
`To date various experimental procedures have been proposed and used to evaluate the
`absorption rate of drugs parenterally administered: (1) observation of changes in the pharmaco-
`logical effect with time;
`following the blood level and/or urinary excretion, and pharmaco-
`kinetic analysis of the data 3 (3) following drug clearance at the injection site {local clearance
`method). We adopted the local clearance method since a sample oily solution injected
`intramuscnlarly remains localized, allowing almost complete recovery of the remaining drug.
`Apparent elimination of the drug observed by the local clearance method sometimes results not
`only from its transport. to the blood or lymph system but also from its metabolic change at
`the injection site. Hence, in order to check the contribution of the latter factor in the case of
`the model compounds, an in viiro incubation experiment with removed muscle tissues" was
`undertaken and the resulting data of 7% recovery for agiven period were compared with those
`from the in viva absorption experiment. Table I gives this comparison for eachconipound
`in the 80 solution. For all compounds, the recovery was near 100% for the in mifro incubation
`experiment but considerably lower for the in nine absorption experiment. Further, no signifi-
`cant change of the visible absorption spectrum was observed before and after these two experi-
`ments. These results showed that the metabolic change of these model compounds in the
`muscle was almost negligible and their apparent clearances observed here resulted mainly from
`transport into vascular systems. These findings support the View that the model compounds
`and the absorption experiment procedure adopted here are appropriate for analyzing the true
`absorption phenomena of practically wateninsoluble drugs injected intramuscularly.
`
`TABLE I. Comparison of % Recovery between in Vireo Incubation with Removed
`Muscie Tissues and in Vivo Absorption Experiment“)
`
`% recovery”)
`Compound
`Time (hr)
`11.: -uitwa
`In viva
`
`
`PI-IAB
`PAAB
`OAAT
`PANA
`Sudan III
`
`2
`2
`5
`5
`B
`
`97.0 (2.9)
`98.9 (5.8)
`99.6 (4.3)
`100.0 (5.7)
`100.5 (0.9)
`
`38.1 (3.7)
`52.9 (5.2)
`65.0 (8.5)
`91.3 (4.2)
`98.8 (0.8)
`
`tr} Vehicle, 50; initial concentration (Cu), 0.55 mglml; injection volume (Va), 0.05 ml.
`b) Each value represents the mean of 3 experiments with the standard deviation in pamritheses.
`
`Clearance of Sesame Oil from the Injection Site
`
`Various vegetable oils such as sesame oil, olive oil, cotton seed oil, peanut oil and castor
`.
`oil are usually used as oily solvents for preparations. Sesame oil (80) is most commonly used
`in japan. Since these oils consist not of a single component but of various triglycerides such
`as olein, linolein, stearin, palrnitinand myristin, and other natural products, direct evaluation
`of their absorption characteristics from the injection site seems impossible. Recently, some
`reports have appeared on intramuscular absorption of mineral oil,“ 14C—methyl oleate,“ and
`14=C—tripalrnitin mixed with sesame oil.“ Ail these investigations indicated that these oils
`are absorbed very slowly.
`An oil vehicle may be eliminated from the injection site mainly via one or both of two
`routes: (a) absorption after being dissolved in the body fluids or transformed into some hydro-
`philic compounds and (in) direct absorption alter division into micro—droplets. Table I shows
`that Sudan III, which could scarcely be released from the 50 Vehicle into the aqueous phase,
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`was insignificantly absorbed for at least 6 hr after injection. Thus, with Sudan III as a tracing
`material, the absorption of S0 was examined by an indirect method, as described in “Experi-
`mental.” The results appear in Table II. The residual volume ( V) ‘of SO and the residual
`amount
`of Sudan III 6 hr after injection had little changed from their initial values, but
`subsequently decreased gradually.
`* This observation showedthat S0 was scarcely absorbed
`during the first several hours after injection, but that after this it might be very slowly
`absorbed, presumably via both routes mentioned above.
`
`TABLE I1. Absorption of Sesame Oil Estimated by the Indirect Method
`
`
`
`
`
`
`
` me (no . W ms)” 0 (train) V ll-.1)»
`
`
`
`53.5 (0.4)
`5.07 (0.01)
`272.0. ( 2.1)
`0
`53.5 (0.2)
`5.03 (0.02)
`255.7. ( 1.0)
`5
`43.5 (1.9)
`5.00 (0.13)
`242.9—( 7.3)
`5
`20
`
`41 45.5 (5.2) 239.4. (24.3) 5.25 (0.28)
`
`
`:1) Residual amount of Sudan III.
`£1) Sudan III concentration in the oily depot phase.
`c) Calculated residual volume of SO. V:W,'C. Each result is given as the mean of at least 5 experi-
`ments icilowed by the standard deviation in parentheses.
`
`Time Course of Drug Absorption and Eifect ofInitial Drug Concentration
`Fig. 1 shows a time course of intramuscular absorption of PAAB after injection of its
`80 solution. The initial concentration (Co) and injection volume (V9) are given in the legend.
`The upper figure (A) shows the value of the ‘X, remaining on a linear scale While the lower one
`(B) uses a logarithmic scale. From the linearity shown in Fig. IB, this compound was expected
`to be absorbed mono-exponentially, that is, according to a 1st order rate process.
`In order
`to confirm this, the effect of the initial concentration (C0) of PAAB in the SO solution on absorp— «
`tion was examined.
`Fig. 2 shows a comparison of time courses of PAAB absorption on
`a semilogarithmic scaleirom three solutions of different C“. All these absorption profiles gave
`
`%remaining Fig. 2. Efifect of Initial Concentration on
`
`%remaining
`
`Time (hr)
`Fig. 1. Linear (A) and Seinllogarithmic
`(B) Plots of ‘A, Remaining PAAB at the
`Injection Site versus Time (SO)
`Each point represents the mean of 4 experiments.
`The vertical bar about the mean shows the standard
`deviation.
`Cg, 5mg/ml; V", 0.06 ml.
`
`'
`
`PAAB Absorption following Intramus-
`cular Injection (SO)
`Key: C), 0.5 mglml: A, -10 mglml; solid line, 5
`mgfml (shown in Fig. 113). Each point represents
`the mean oi at least 3 experixn ants.
`T/.,: 0.05 ml.
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`straight lines with the same slope, not depending on C0. Other model compounds such as
`PHAB, OAAT and PANA in 80 solution also showed similar behavior. These results were
`analogous to those obtained by Tanaka et al.“ who studied slightly Water~SoIuble drugs and
`oil systems. Others have also reported that intramuscular absorption of neutral drugs in
`. aqueous solution, different from that of acidic or basic drugs, depended little on their initial
`—c0ncentrati0n.2“~1"’ Therefore, it seems reasonable to conclude that intramuscular absorp-
`tion of neutral and practically wateninsoluble drugs in water—immiscible oil solutions obeys
`approximately 1st order kinetics.
`
`Effect of Injection Volume
`
`Injection volume is one of the important factors which influence drug absorption. Fig. 3
`shows the efiect of the injection volume on the absorption time profiles following intramuscular
`administration of PAAB in S0. The slope of these profiles increased gradually with decreasing
`injection volume. The quantitative relationship is discussed later.
`
`Time
`
`2
`
`4
`
`5
`
`3
`
`(hr)
`
`as 20
`
`E” 5“
`‘E 40
`E
`5-:
`
`1000
`80
`
`-S
`E
`5..
`as
`
`Injegtion Volume on
`Fjg_ 3_ 7 Effect of
`Intramuscular Absorption of PAAB from
`50 5°1“*i°'1
`_.
`_l
`,_ v 10 1, _
`..
`l, _
`:_ _
`I-
`#]?:ac1(1)pbii1ItAnepi%sdnts the min tit
`nl'KfiyQ— Yml
`4 or 5 experiments. C9: 5 mgfrnl.
`
`Intramuscular
`Fig. 4. Comparison of
`Absorption of PAAB from Va.ri01zs'Oily
`Solutions
`Kev: -3-. DES; —o—. Miss -v—. co; <>—. IPM;
`-0- S0. Each giointrepresents thernean of 40: 5
`°xPe’:immt5'
`69’ 5 mgtmh V“ 0'05 mt‘
`
`Comparison of Absorption Rates from Various Oily Solvents
`In parenteral preparations for animal experiments, oily solvents other than sesame oil
`are used with or without some adjuvants. Fig. 4 compares the intramuscular absorption
`rates of PAAB in five oily solvents, DES, Mig, C0, IPM and SO. Each semilogaritlunlc plot
`was nearly linear with a different slope. Similar results were also obtained for other com-
`pounds, PHAB and OAAT. The major factors which resulted in the remarkable differences in
`the absorption rate from different vehicles will be discussed below.
`
`Discussion
`
`Kinetics of Drug Absorption
`Visual observation throughout all our absorption experiments confirmed that dye solution
`injected intrainuscularly was confined to the planes of fascia or connective tissues surrounding
`muscles, did not spread as extensively along the fascial septums as did the aqueous systems,
`and formed a depot which took a flat shape similar to a pod. The geometrical change of the
`oily depot was more abrupt within a short period during and immediately after injection but
`gradually became less with time. Our observations were similar to those of Scaffenn’
`When a practically wateninsoluble drug in a water—immiscible oil solution is injected
`intrarnuscularly, two possible routes for the absorption of the drug component may be con-
`sidered: ('i) absorption by direct transport of small oil droplets containing the drug and (£13)
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`absorption by the same mechanism as an aqueous solution after drug release from the oily
`phase into the aqueous phase. The-absorption data for the oily solvent (SO) shown in Table
`II imply that for several hours after injection, 80 was scarcely absorbed. Thus, the second
`route (ii) is expected to be the major one for drugs which are more quickly absorbed than the
`oily solvent.
`Thus, assuming the driving force for drug transport to be diffusion, a model was proposed
`to clarify the absorption process kinetically. Fig. 5 shows the absorption process of the drug
`component diagrammatically with a two—dimensional model.
`.The broken lines represent
`the concentration gradient along the distance axis perpendicular to the interface (i) between
`the oily and aqueous phases. First, the drug molecule is transported in the oily phase (I)
`to the interface (i), where it is then distributed into another phase (II).
`Subsequently, the
`drug molecule in the aqueous phase (II) diffuses with or without the aid of body fluid flow
`through the biological matrix, including cell membranes or intercellnlar spaces of connective,
`muscle and vascular tissues, into the blood or lymph stream. The shape of the oily depot
`in the injection site may fluctuate continually in response to the motion of the surrounding
`muscles and this may cause some convection flow, that is, agitation Within the oily depot
`phase
`In the aqueous phase (II), the flow of body fluid is expected to be vigorous, espec-
`ially in the region adjacent to vascular systems (III).
`If we assume on the basis of these
`considerations that the rate-limiting process of drug absorption may exist in the diffusion
`in both phases close to the interface (1), the fluxes of the drug molecules -per unit time, f0 and
`fl, are given by the following equations according to Fick‘s diffusion law:
`
`In = —D.. (
`
`)Hl
`
`<o<X<Xa
`
`fa — D». ( ,X)H1
`
`cX>Xo
`
`(sq. 1)
`
`(Eq. 2)
`
`where D, and D, represent diflusion coefiicients in the oily and aqueous phases, respectively,
`and (BC/BX) signifies the concentration gradient of the drug in the oily (O<X<X,) or aqueous
`I (X>X,) phase. Assuming that the thickness of the diffusion layer. may be approximately
`regarded as constant, Eqs. 1 and 2 become‘:
`In =ko(Cnll—Clol1)
`In == kn(C'u-Cb)
`
`(Eq. 3)
`(Eq- 4)
`
`-
`
`where Fe, and k, are defined as parameters related to transport in the oily and aqueous phases,
`respectively, and C0,, and C, repesent concentrations in the oil bulk phase and blood, respec-
`
` 3
`
`5
`
`50 70100
`
`30
`7 10
`.
`i
`V‘) (#1)
`Fig. 6. Relation between Absorption Rate
`Constant (Ia) and Injection Volume (V0)
`Key: -0-, intact, m. gastracnemius (C3,, fimgfml);
`-<>-, intact, m- miusfzmoris (Ca.10ms.'m1);
`-A—,.
`anesthetized, m. gastrncncm-.'us (Cu, 10 mg/ml).
`Test compound, PAAB; vehicle, 50.
`
`Fig. 5. Model for Intramuscular Drug
`Absorption from Water-immiscible Oil
`Vehicle
`I : Oily depot phase,
`ll’ 1 Aqueous bD<5Y fluid phase containing various tis-
`sues adjacent to oily depot,
`JJI: Blood or lymph.
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`
`tively; C ’on and C ’,, mean the concentrations in the oily and aqueous phases at X =X,, respec~
`tively. Under the condition that C’,,>>C,,, Eq. 4 can be approximated as:
`
`J, = k.;C’a
`
`(Eq. 5)
`
`Assuming that the distribution equilibrium of the drug molecule at the interface is reached
`quickly compared with its transport, the distribution coefficient (Kg at the interface is defined
`by
`
`K1 == C’cmfC’a
`
`‘
`
`5
`
`i
`
`6)
`
`After a quasi-steady-‘state is reached at time r, the following relationship is established:
`
`—dW/db‘ = Ale = Afr.
`
`05>?)
`
`039- 7)
`
`where W represents the amount of drug in the oily depot phase and A is the surface area of
`the oily depot. W is given by the volume of the oily depot, V, and the drug concentration
`in it, CD”, according to the following equation:
`'
`
`W = Cow '
`
`C1
`From Eqs. 3, 5, 8, 7 and 8, the following e nation can be derived:
`
`.5911 r: M ___..’i‘:i%...~«n W
`‘”
`V(i+~°—Ki)
`L
`:1
`
`(Eq. 8)
`
`(Eq. 9)
`
`Since the absorption of oil itself is very slow compared with that of the drug component, as
`shown in Tables I and II, V can be regarded as equal to the injection volume V" for several
`hours after administration. Under the condition that the change in the surface area of the
`oily depot, A, is sufiiciently small, Eq. 9 shows the 1st order kinetics and can be integrated.
`When lag time t is sufficiently small compared with the absorption time scale, the foilowing
`relations are readily derived:
`
`in(W/Wu) : -12:
`
`k = ?:L1—:
`
`(Eq. 10)
`
`11)
`
`where W0 represents W at 15:20, that is, the dose, and la is defined as a lst order absorption rate
`constant. For the model drugs used in this study, it was observed that a large portion of the
`amount remaining in the injection site Was localized within the oily depot phase. Accordingly,
`the value of W_/W‘, for these drugs is nearly equal to the residual fraction determined by the
`absorption experiment described above.
`t
`The experimental results shown in Fig. 2 indicate that the absorption obeys 1st order
`kinetics. Therefore, provided that the assumptions and conditions in the above consideration
`based on the model are valid, the apparent 1st order absorption rate constant experimentally
`obtained is‘ governed by A, Va, kg, k, and K1..
`
`Relationship between Absorption Rate Constant and Injection Volume
`
`Some papers have reported that the intramuscular absorption rate of a drug in aqueousmam
`or oily solution"”'might decrease with increasing injection volume. However, the quantitative
`relationship between absorption rate and injection volume or the influence of anesthesia and
`the injection site on it has not been completely elucidated. Fig. 6 shows a plot of the 1st
`order absorption rate constant (is) against injection volume (T/'0) on a log~log scale.
`In this
`figure, the data obtained froIn'Fig. 3 are indicated by open circles. This plot gave a straight
`line with a slope of -0.14 (correlation coefficient, i*:-0.95). The data from intramuscular
`injection into the other site, the m. reams famoris, in intact rats (indicated as open squares
`in Fig. 6} gave an almost identical straiglitjline.
`In anesthetized rats, as shown by open.
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`3
`
`‘
`
`527
`
`triangles in Fig. 6, the absorption rate was remarkably slower compared with the corresponding
`data in the unanesthetized (intact) rats. A plot of log k versus log V0 gave a similar straight
`line but with a larger slope, —0.32 (r=——O.97].
`Eq. 11 means that the absorption rate constant is is proportional to the term A /V9, which
`depends on the injection volume (V0). The term A ] VB is proportional to 'V0—1/3 when the
`oily depot retains a similar shape or dimensions for the change in V0, and it is constant when
`the oily depot expands two-dimensionally at a constant thickness with increasing V0. The
`relation fa cc V9432 observed in anesthetized rats may correspond to the former case, while
`k 05 V0-0'14 observed in unanesthetized rats may correspond to an intermediate case between
`the two cases mentioned above.
`It seems possible that in intact rats, unlike anesthetized
`rats, the oily depot cannot retain a similar shape or dimensions for an increase in V0 because
`of the mot