.Phormaceutical Research, Vol. 9, No. 1, 1992
`
`Report
`
`Kinetics of Degradation of
`Levothyroxine in; Aqueous Solution
`and in Solid State
`
`Chong Min Wonl
`
`Received January 23, 1991; accepted August 27, 1991
`The kinetics of the deiodination of levothyroxine in aqueous solu-
`tion were studied over the pH range 1 to 12. Temperature depen-
`dence .of the reaction was also studied. The log k (cid:9)
`pH profile
`indicated that the kinetics of deiodination include proton attack on
`the anion and .dianion in acidic sohition and water attack on the
`anion and dianion in alkaline solution. A possible mechanism of the.
`.deiodination was discussed:The 'solid-state xtability of levothyrox-
`ine sodium was studied at elevated. temperatures; and the compound
`was found to undergo deamination on heating. The decomposition
`follows biphasic first-order kinetics, with the most rapid decompo-
`sition occurring at the beginning of heating.
`KEY WORDS: levothyroxine; liothyronine; degradation, deiodina-
`tion; pH effect; solid-state stability.
`
`INTRODUCTION
`
`• LeVothyroxine sodium is the sodium salt of the levo-
`isomer of thyroxine (T4), an active physiological compound
`found in the thyroid gland. Preparations oftheSYnthetic horl
`nione are indidated in replacement or supplegiental therapy-
`for patients with hypothyroidism and as 'pituitary thyroid
`stimulating hormone suppressants in the treatment of etithy-
`roid goiters (1). (cid:9)
`'
`. (cid:9)
`,
`Deiodination occurs- as a resultof ultraviolet irradiation
`in aqueous media, and the deiodination process was propor-
`' tional to the decrease in the pH of the aqueouS solution (2).
`When T4 labeled with '31I was administered intravenously to
`normal htimans, most labeled iodine eventually appeared in
`the urine as free inorganic iodide (3). The possible physio-
`logical significance of the deiodination . of the thyroid hor-
`mone caused many investigators to study the process both in
`vitro and in vivo. The deiodination of T, occurred in kidney,
`brain, liver, and muscle homogenate preparations. There
`was, however, only limited evidence for liothyronine (T3)
`formation (4),
`Despite a long history. of its use, the in vitro degradation
`studies of T4 appearing in the above literature were of de-
`scriptive nature, providing limited kinetic information. The
`purpose of this study was to obtain kinetic data on the deg-
`radation of T4 in solution and in solid state and to investigate.
`possible mechanIsms of the degradation 'processes. This
`Study was, not intended to predict the in vivo transformation
`of the dnig.
`
`' Analytical and Physical Chemistry' Department, Rhone-Poulenc
`Rorer Central Research, 500 Virginia Drive, Building No. 5, Fort
`Washington, Pennsylvania 19034.
`
`MATERIALS AND METHODS
`
`Materials
`
`Levothyroxine sodium and liothyronine were obtained
`'from Biochemie (Austria) and Aldrich Chernical (Milwau-
`kee, WI), respectively. Water used for kinetics was deign-
`ized and distilled. Acetonitrile used was - HPLC grade. All
`the chemicals used were reagent grade and were used as
`received. TLC plates, were obtained from Analtech (New-
`ark, DE).
`
`HPLC Analysis
`
`The chromatography system consisted of an isocratic
`pump (Beckman Model 110A); an automatic injector (IBM
`LC/9504), a variable-wavelength UV detector (LDC Spec-
`tromonitor III), and a computing integrator (Hitachi D-2000). -
`The HPLC method eMplOyed a 250 x
`particle size, cyan-bonded silica column (Phenomenex) and
`a mobile phase consisting of wateracetonitrile:phoaphoric
`acid (600:400:1, v/v). The flow. rate was 1.5 ml/min and the
`detector wavelength for. ultraviolet absorbance detection
`was 225 nm. (cid:9)
`•
`
`Solution Degradation Product .
`
`Approximately 500 rag of levothyroxine sodium was dis-
`solved in 70 ml of 0.01 N NaOH and refluxed for 40 hr. The
`solution was then neutralized by adding concentrated HCI
`dropwise and evaporated to dryness using a-rotary evapora-
`tor. The residue was extracted-with 50 ml of acetonitrile. The
`acetonitrile solution was concentrated, -streaked across a
`14mm-thick silica get OF. preparative TLC plate, and devel- •
`oped with a solvent system consisting of methylene chloride: .
`acetonitrile:formic acid (80:10:10). Using the TLC system, a
`T. spot showed at Rf 0.27 and a, major degradate spot at- RI
`0.18. The band corresponding to /21.0.18 was removed front
`the plate and eluted with 50 ml of 0.01 N methanolic NaOH
`solution. The eluent was acidified by adding concentrated
`HCl dropwise and the'resultant precipitate was centrifuged.
`The supernatant was decanted and the solid was dried under
`vacuum at 60°C for 2 hr.
`The electron impact mass spectrum' of the degradate
`was run on a VG 7070 SE mass spectrometer. The spectrum
`was obtained at 70 eV with the source temperature at 190°C.
`'The low-resolution mass spectrum exhibited a protonated
`molecular ion at m/z .651 corresponding to C15H12I3N04, loss
`of water (m/z 633), and loss of carbon dioxide (m/z 607). The
`proton NMR spectrum of the isolated degradate was re-
`corded on a Varian VXR 200 NMR spectrometer using deu-
`terated DMSO as the solvent. The proton NMR spectrum of
`the degradate showed a doublet (6.91. and 6.95 ppm) corre-.
`sponding to the hydrogen atom which had replaced, iodine at
`either the 3', or the 5' carbon, a doublet (7.02 and 7.03 ppm),
`and a quadruplet (6.60, 6.62, 6.65 and 6.66 ppm), corre-
`sponding to the hydrogens at 2' and 6' carbons. The MS and
`NMR spectra were identical to those of authentic T3. The
`major degradation peak seen in an HPLC chromatogram of •
`T4 which had been partially degraded in acidic solution had
`a retention time identical to that of authentic Ts. The coin-
`
`07244741/320100-0131506.500 0 1992 Plenum Publishing tarporaloo
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`Mylan Ex 1017, Page 1
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`

`

`1
`
`Scheme I .
`
`ficients (5). In a typical kinetic experiment, 0.5 ml of 1 mg/m1
`levothyroxine sodium stock solution in methanol, an appro-
`priate amount of buffer stock solution, HCI or NaOH, and an
`appropriate amount of 1 M NaCI were transferred into a
`100-m1 volumetric flask and filled to volume with water. The
`final concentration of levothyroxine sodium was .5 ug/ml.
`The reaction flask was kept in a constant-temperature water
`bath at 50.0, 60.0, 70.0, and 80.0°C (±0.5°C). Aliquots of the
`sample were taken at appropriate time intervals and ana-
`, (cid:9)
`.
`lyzed by HPLC. (cid:9)
`
`RESULTS AND DISCUSSION
`
`Solubility—pH Profile
`
`T, has three ionizable moieties and it can exist as the
`cation (C), zwitterion (Z), anion (A), and dianion (DA), de-
`pending on the pH of the solution (Scheme I). The solubility—
`pH profile (Fig. 1) was used to determine the ionization con-
`
`jection 'of an acid-degraded T4 with authentic T3 resulted in
`one peak (chromatogram not shown).
`
`Solid-State Degradation Products
`
`Approximately 500 mg of levothyroxine sodium in an
`open vial was kept in an oven at 60°C for 7 weeks. The.
`degraded levoihyroxine sodium was suspended in 150 ml of
`a methanol and water mixture (30:70). The sample was com-
`pletely dissolved by dropwise addition of cone. NH,OH and
`subsequently acidified, by dropwise addition of conc. Ha.
`The resultant precipitate was filtered, dissolved in methanol,
`streaked across a 1-mm-thick silica gel GF plate, and devel-
`oped with a solvent system consisting of toluene:ethylace-
`tate:acetic acid (75:22:3). Using the TLC system, a T, spot
`showed at R10.05 and two major degradate spots at R10.89
`and 0.06. The bands corresponding to Rf 0.89 and Rf 0.60
`were separately removed from the plate and eluted with
`methanol. The eluent was evaporated to drynesk and the
`residue was dried at 60°C for 2 hr under vacuum. High-
`resolution EI-MS of the Rf 0.89 product showed a molecular
`ion. at m/z.717.6488 corresponding to. Ci3H6031, (calculated
`m/z 717.6494) and ions resulting from the two subsequent
`losses of iodine (m/z 591 +. H, 464). The other product (R,
`0.60) showed a protonated molecular ion at m/z 760.6638
`corresponding to C,51-191,04 (calculated m/z 760.6677) and
`ions resulting from loss of water (m/z 742) and loss of carbon
`dioxide (m/z 716).
`
`. Solubility—pH Profile
`
`•
`The solubility of levothyroxine sodium in aqueous
`buffer solution was determined as a function of pH ranging
`from 1 to 11. The buffers used were glycine (pH 1.0-3.7),
`acetate (pH 4.5-4.7), phosphate (pH 5.8-8.0), and carbonate
`(pH 9.3-11.0) buffers. The ionic strength was adjusted to 0.5.
`An excess amount of levothyroxine sodium was placed in a
`Screw-cap test tube containing the aqueous buffer solutions.
`The solution was shaken at 25°C on a rotary shaker until
`replicate analyses of the solution sampled at 4-hr intervals
`indicated that .a steady concentration had been reached.
`Eighteen hours of shaking sufficed for the equilibration. The
`undissolved solid was then separated by centrifugation. The
`pH of the solution was measured, and after appropriate di-
`lutions with the mobile phase, the sOlution.was analyzed by
`HPLC.
`
`• Kinetic Method
`
`The buffers used for kinetics were citrate (pH 3.2). ac-
`etate (pH 4.4-5.4), phosphate (pH 6.3-7.0), borate (pH 8.8),
`and carbonate (pH 9.6-10.7) buffers. Buffer stock solutions
`of 0.2 M were prepared. Aliquots of the buffer stock solu-
`tions and 1 M NaCI. solution were diluted so that the final
`total buffer concentration was 0.02 Mend the ionic strength.
`was at 0.1. Low buffer concentrations were used to minimize
`the possible, general acid—base catalysis by the buffer spe- (cid:9)
`cies. The pH of each solution was measured at the reaction
`temperature. For strongly acidic and basic solutions, aque-
`ous HCI or NaOH solutions were used to obtain the desired
`pH. The pH values of these solutions at the reaction tem- (cid:9)
`peratures were calculated from the published activity coef- (cid:9)
`
`-0.
`
`I (cid:9)
`
`2 (cid:9)
`
`3 (cid:9)
`
`• (cid:9)
`
`S (cid:9)
`
`7 (cid:9)
`
`91 (cid:9)
`
`9 (cid:9)
`
`10 (cid:9)
`
`12
`
`Plot of logarithm of solubility of T., as a function of pF1'at
`25°C.
`
`Mylan Ex 1017, Page 2
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`

`

`Kinetics of Degradation of Levothyroxine
`
`133
`
`scants. (6). The equation for the solubility (S) of ts as a func-
`tion of, hydrogen ion activity up to pH 9.8 is given by
`• -
`S = So (I + [11')/K,,, + Ica/MI) (cid:9)
`(1)
`where So is the intrinsic solubility of Z. The value of So (0:25
`p.g/m1) was estimated from the best fit of the solubility—pH
`profile between pH 3 and pH 6. The pK„ values that gave the
`best fit to Eq. (1) are pKa, = 2.40 for. the carboxyl group and
`pK.2 = 6.87 for the phenolic OH group. The solubility—pH
`profile shows a break around pH 10 indicating a saturation
`
`solubility of A. As the pH increases above 10, the solubility
`increases as more DA is formed, in accordance with
`+ So' (1 + K,V[1-11) (cid:9)
`S (cid:9)
`(2)
`where So' is the maximum solubility of A. The So' (120 14,/
`m.1) and plc3 (9.96) values were estimated from the best fit of
`the solubility—pH profile above pH 10. The plc values de-
`termined under these conditions are slightly different from
`the published values of 2.2 for the carboxyl group, 6.7 for the
`phenolic group, and 10.1 for the amino group (7).
`
`100
`
`C
`
`o.
`
`m•
`CC
`
`to
`0 (cid:9)
`
`100 (cid:9)
`
`NO (cid:9)
`
`140 - (cid:9)
`
`150 (cid:9)
`
`CIO
`
`Time (Hours)
`Fig. 2. Typical fzst;order plots for the deiodination of T4 in aqueous solution at 80°C. pH 2.05 HP; pH 6.136 (6):
`pH 7,96 (A); pH 10.55 (•).
`
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`

`

`•
`
`e•tentioa
`fa (11111•0 (cid:9) "
`Fig. 4. Typical HPLC chromatograms of partially degraded sam-
`ples. (a) T4 in acidic solution."(b) T. in alkaline solution. (c) T, in
`acidic solution. (d) T, in alkaline solution. T. (A); T3 031: (cid:9)
`(C);
`unknown (D). (cid:9)
`•
`
`amino group (pKA3 - 9.96) was ionized. The carboxyl and
`amino groups are too far away from the reaction center so
`that the ionization of these groups appears to have little res-
`onance or inductive effect'on the deiodination rate. •
`As shown in the log k - pH profile, the rate constant
`does not increase with increasing acidity in the strongly
`acidic region, indicating that there is no contribution to the
`rate constant from the proton attack on C and Z. The overall
`deiodination of T4 can be explained by apontaneous or wa-
`ter-catalyzed reactions (ke) of C and Z and spontaneous or
`water-catalyzed reactions (ke') of A and DA.. The deiodin-
`ation rate constant can be formulated as
`k = ke f.A + ko f A (cid:9)
`(3)
`where /HA atidfA are the sum of the fractions of C and Z and
`of A and DA, respectively. Equation (3) is kinetically equiv-
`alent to
`
`k = (cid:9)
`(4)
`[141 fA ko fA . (cid:9)
`where ke = kH Ka and kH is the catalytic rate constant for
`proton attack on A and DA. Substituting/A = Ka/([H*] +.
`Ka) in Eq. (4),
`
`6,-91 -..0011
`'
`
`2 (cid:9)
`
`2 (cid:9)
`
`• (cid:9)
`lo (cid:9)
`ll (cid:9)
`IZ (cid:9)
`12
`Fig. 3.. log k - pH profiles for the degradation of T. in aqueous
`solution at 50°C (0), 60°C (A), 70°C 0), and 80°C (0) and of T, at
`80°C RE.
`
`Lug k - pH Profile
`
`For all the kinetic experiments, the first-order plots for
`loss of T4 were linear (correlation coefficient >0.998) for two
`or more half-lives. Typical first-order plots are shown in Fig.
`2. The log k - pH profiles for. the deiodination of T4 are
`shown in Fig. 3. The rate of deiodination shows a plateau in
`the acidic pH region, drops off sigmoidally in the neutral pH
`region, and shows another plateau in the alkaline pH region.
`The log k -. pH profile shows a sigmoidal change only when
`the phenolic OH group, is ionized (p/Ca = 6.87). No change
`- was observed when the carboxyl group (pK," = 2.40) or
`
`Table 1. Catalytic Rate Constants,' Arrhenius Parameters,' and En-
`tropies of Activation for Deiodination of T.
`
`• k„
`(sec (cid:9) M'')
`
`Temperature
`• ("C)
`80.0 It. 0.5
`70.0 ± 0.5
`60.0 ± 0.5
`50.0 ± 0.5
`25.0'
`E. (kcal • mol-
`log A
`AS* (cal, molt K-1)
`° Where k =
`(Hi (cid:9)
`keV4•
`Where log k = -4/2.363 RT + log A.
`ExtrApolated from.E, and log A. (cid:9)
`
`14.3
`7.00
`3.16
`1.56
`0.165
`16.8 (cid:9)
`,
`11.5
`
`10' ke'
`(sec '')
`
`
`34.3
`20.8
`13.0
`6.95
`1.36
`12.1
`1.0
`-55.9
`
`NI-CH-COON Cr)).
`
`Scheme II
`
`Mylan Ex 1017, Page 4
`
`(cid:9)
`(cid:9)
`

`

`Kinetics of Degradation of Levothyroxine
`
`135
`
`k = (cid:9)
`
`[Hi + koll([1-11 + K.,2) (cid:9)
`• (5)
`In the acidic pH region, ili+) S. K,2, and Eq. (5) reduces to
`k= kH K.2 ' (cid:9)
`
`(6)
`
`In the alkaline pH region, [11-1 4 K.2, and Eq. (5) reduces
`to
`
`k = ko (cid:9)
`
`(7)
`The catalytic rate constants, kH and ko', were estimated from
`the best fit of the log k — pH profiles (Table I). The Arrhe-
`nius parameters of the rate constants are included in Table I.
`
`•: (cid:9)
`
`Mechanism of Deiodination
`
`The HPLC chromatogram (Fig. 4a) of T. partially de-
`graded in acidic solution contains a peak with a retention
`time corresponding to that of T,. The HPLC chromatogram
`(Fig. 4b) of T. partially degraded in alkaline solution con-
`tains a peak with a retention time of 4.0 min , (Peak D) of an
`' (cid:9) unknown degradate but no peak corresponding to that of T5.
`When the initial concentration of T4 was increased above 5
`metiril in alkaline degradation, T, was found to be the major
`'degradate. The chromatogram (Fig. 4c) of T3 partially de-
`graded in acidic solution contains a peak with a retention
`time corresponding to that' of 3,5-diiodothyronine (T2). The
`chromatogram (Fig. 4d) of T, partially degraded in alkaline
`solution contains Peak D. (cid:9)
`•
`, When degraded under the same conditions as T. at
`80°C, T. was found to be more stable than T4 in acidic so-
`lution and less stable in alkaline solution (Fig. 3): In alkaline
`solution,, because T3, was found to be less stable than T. and
`because T3 was the major degradate at higher 14 concentra-
`tion, it can be assumed that T. degrades to T3 and Ts de-
`grades further to the unknown degradate (Peak D) as soon as
`• it is produced. At higher T4 concentrations, the high con-
`. centration of free iodonium ion in solution may play an im-
`. portant role in the deiodination rate (8).'At higher iodonium
`ion concentrations, the deiodination may follow a different
`rate law, and T, may become relatively stable.
`. The usual mechanism of the iodination of phenol is a
`two-step process involving an addition. of an iodonium ion to
`the carbon, followed by a loss of the hydrogen atom being
`replaced (9). According to the principle of microscopic re-
`versibility, the reverse sequence of these steps constitutes
`the favored mechanism for the reverse reaction, deiodin-
`ation.
`Among the four iodines in the T. molecule, those at the
`3' and 5' positions are more labile than those at the 3 and 5
`positions (3). The substituted phenoxide ion moiety gives 3'
`and 5' carbons carbanionic character by resonance so that
`these carbons are favored for electrophilic attack by proton
`or other electrophiles (Scheme II).
`, (cid:9)
`Even though' Eqs. (3) and (4) are kinetically equivalent,
`they imply different mechanisms for T. deiodination. A sim-
`ple pH. dependency study does not allow differentiation of
`the two equations and the possible mechanisms consistent
`with these equatiOns. However, because a proton should be
`a far more powerful electrophilethan water and because the
`substituted phenoxide ion should be vastly more reactive
`than the substituted phenol, it is very likely that the proton-
`
`CW.CMC00...
`
`Scheme III
`
`ation is due to proton attack on the anion and dianion in
`acidic solution rather than water attack on the cation and
`zwitterion. The protonation in alkaline 'solution may be due
`'to water attack on the anion and dianion. Therefore; the
`reaction scheme consistent with Eq. (4) is a more likely
`mechanism than that defined by Eq. (3).
`
`Solid-State Degradation
`
`Low assay values area concern of most manufacturers
`of levothyroxine sodium products, and in all probability, it-
`might be attributable to solid-state instability of the drug
`substance (10).
`
`In Contrast to its solution degradation, levothyroxine
`sodium does not deiodinate in solid state. Rather, the iso-
`lated degradation products indicate a deaminatiOn reaction
`(Scheme. III). Plots showing the rates of solid-state degrada-
`tion are presented in Fig. 5. The solid-state degradation ex-
`hibited biphaiic first-order degradation profiles indicating
`the possibility of complex degradation pathways. Correla-
`tion-coefficients of no less than 0.994 were obtained for lin-
`ear portions of the first-order plots. The initial rate of deg-
`radation is much greater than that obtained in the later part
`of the degradation curve at all temperatures. The rapid deg-
`radation phase is more pronounced at higher temperatures.
`As the temperature is lowered, the initial phase becomes
`shorter compared to the later phase. The initial phase is
`almost nonexistent at 50°C. It appears that there is a thresh-
`old temperature between 50 and 60°C where levothyroxine
`sodium degrades rapidly. The biphasic degradation kinetics
`of levothyroxine sodium can be described by
`
`D = 13„ (A e -4' B (cid:9)
`
`- (cid:9)
`
`(8)
`
`Table O. First-Order Rate Constant for Degradation of T. in Solid
`State in Accordance with D = D,, (A e-4. 4- B e-41'
`
`Temperature CC) (cid:9)
`50 ± 2
`60 ± 2
`70 ± 2
`80 it 2 • (cid:9)
`
`k, (sec-') (cid:9)
`
`10' kb (see
`
`7.19
`
`1.71
`3.97
`7,47
`
`Mylan Ex 1017, Page 5
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`

`

`136
`
`Won
`
`i0 It 14 10 as so se 24 28 28 38
`
`rime (DayB)
`Fig. 5. Solid-state degradation of levothyroxine sodium at 50°C (41), 60°C (A), 70°C (1)), and 80°C (M).
`
`where D. and D are the drug amounts of levothyroxine so-
`dium at time 0 and r, respectively, A and ic are 'the intercept
`and slope of the rapid phase, and B and kb are the intercept
`and slope of the slow phase. For practical purposes, the kb
`Value in the rapid phase is more critical'than the kb value in
`the slow phase, since the former contributes more to the first
`10% T4 loss. Unfortunately, it was not possible to determine
`the initial rate of degradation at lower temperatures with
`reasonable accuracy, because the initial rapid degradation
`phase is shorter at lower temperatures. The first-order rate
`constants obtained by feathering (10) are listed in Table II.
`
`The initial degradation rate at 80°C is approximately 10 times
`faster than that of the later phase.
`
`ACKNOWLEDGMENTS
`The author thanks Ms. C. Strohbeck for NMR spectra,
`Dr. S. Kelly and Mr. J. deKanel for mass spectral analysis,
`and Dr. C. Obetz for helpful comments.
`REFERENCES
`
`I. E. R. Barnhart. Physicians' Desk Reference; Medical Econom-
`ics, Oradell, NJ, 1989, pp. 1830-1831.
`
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`

`Kinetics of Degradation of Levothyroxine
`
`137
`
`2. A. L. Reviczky and S. B. Nagy. Ontersuchung der Thyroxinde-
`Iodisation mit instrumenteller analytischer Methods. Endokri•
`nologie 56:81-91 (1970). (cid:9)
`• -
`3. J. B. Stanbury. Deiodination of the iodinated amino acids. Ann.
`N. Y. Acad. Sci. 86 (Part 2):417-439 (1960).
`4. T. Jahn, G. 'Morreale de Escobar, and F. Escobar del Rey.
`Pitfalls in studies of in vitro deiodination of thyroxine. Endocri-
`nology 78:7-15 (1966).
`5. H. S. Hamed and B. B. Owen. The Physical Chemistry of Elec-
`trolyte Solutions, Reinhold, New York, 1958, p. 716. .
`6. B. H. Lippold and J. F. Lichey. Loslichkeits-pH Profile mehr-
`protoniger Arzneistoffe am Beispiel des assoziierenden Dimet.
`indent and des zwitterionischen Liothyronines. Arch. Pharm.
`(Weinheim) 314:541-556 (1981). (cid:9)
`•
`7. A. Post and R. J. Warren. Sodium levothyroxine. In K. Florey
`
`(ed.), Analytical Profiles of Drug Substances. Vol. 5, Academic
`Press, New York, 1976, pp. 226-281. .
`B. E. Gravenstein, Jr., and N. S. Aprahamian. Aromatic Haloge-
`nation. II. Kinetics and mechanism of iodination of 4-nitrophe-
`nol and 4-nitropheno1-2,6-4 J. Am. Chem. Soc. 84:212-220
`(1962).
`9. 1. Hine. Physical Organic Chemistry, McGraw-Hill, New
`York, 1962, p. 358.
`10. J. F. Brower, D. Y. Toler, and ). C. Reepmeyer. Determination
`of sodium levothyroxine in bulk, tablet, and injection formula-
`tions by high-performance liquid chromatography. J. Pharm.
`Sci. 73:1315-1317 (1984).
`11. A. A. Frost and R. G. Pearson. Kinetics and Mechanism,
`Wiley, New York, 1961, pp. 162-164.
`
`-5
`
`6
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