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`
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`76
`
`JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
`
`the MIC’s increase in the order: butyl < propyl < ethyl < methyl and the ratios, as
`would be predicted, are roughly the ratios of the solubilities.
`In cosmetic ingredients such as vegetable oils, the solubility order is the reverse of that
`in water; methyl paraben is least soluble and therefore should be the most efhcient
`preservative for oil-rich emulsions. Evans (3) showed that for simple oil/water mixtures
`the best preservative may be propyl paraben at low oil/water ratios or methyl paraben
`at high oil/water ratios but that methyl/propyl mixtures are less efficient
`in both
`cases.
`
`There seems to be a contradiction here between theory and practice. Parabens are
`almost always used in combinations in preserving cosmetics. A search oftheliterature,
`however, yielded no data unequivocally showing synergism in either aqueous broths or
`complex products.
`In our own experimental work we first attempted to demonstrate the applicability of
`the Ferguson principle to the parabens in simple, well-defined aqueoussolutions as a
`step toward resolving the question ofthe utility of mixtures and also to support our
`theoretical proposal
`that single parabens be selected according to solubility. The
`earliest of these experiments (7) showed that the parabens do not follow the Ferguson
`principle to a useful extent; at saturation their antimicrobial potencies are not equal. In
`fact, they drop sharply in the order: methyl > ethyl > propyl > butyl (and benzyl
`paraben, not a member of the homologousseries,is less potentyet).
`The ranking of the parabens is evident from the way the survival curves of E. coli
`change as the inoculation level and saturation fraction are varied. At levels of 10° per
`mlor less the curves are roughly log-linear with about the same slope for methyl, ethyl
`and propyl parabensat saturation; the bacterial population is extinguished in a day or
`two and no survivors are detected thereafter for as long as three weeks.
`The Ferguson principle is clearly applicable under these conditions. With methyl
`parabenat saturation the survival curve remains log-linear to extinction with the same
`slope, as the inoculationlevel is increased to over 10’ per ml; as its saturation fraction is
`decreased the rate of kill decreases butkill is persistent and appears to be complete in
`all cases uncil the saturation fraction is reduced to less than one-half, where the initial
`slope of the survival curve approaches zero. With propyl paraben theinitial kill rate at
`saturation remains the sameas the inoculation level is increased butat levels of about
`10° per ml the survival curve becomes concave up within hours of inoculation and in
`somecases it passes through a deep minimum followed shortly by regrowth at about
`the same rate as in the unpreserved control. Ac still higher inoculation levels the
`minimum is shallow and occurs so early that the initial killing phase (if it occursatall)
`is not detected and only a lag relative to the unpreserved control is noticed. The
`performance ofethyl paraben is intermediate but qualitatively more similar to that of
`propyl paraben:
`the transition from: persistent kill
`to the kill-minimum-regrowth
`pattern occurs but it takes place at higher inoculation levels and lower saturation
`fractions than with propyl paraben.
`We found the same paraben ranking in experiments with Pseudomonas aeruginosa
`ATCC #9721 but with this organism the superiority of methyl paraben is much more
`striking; at saturation it extinguishes inoculations as high as 10’ per mlin less than one
`day while the ethyl and propyl esters cause only transient reductions in survivor Counts
`at inoculations as low as 10° per ml.
`
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`THE PARABENS
`
`77
`
`Because ofits strong dependence on both the inoculum size and the solubility of the
`paraben we first thought that regrowth might be due to depletion of the preservative
`because of its partitioning into the cytoplasm of both the declining number of
`survivors and the growing volume of dead bacteria. We had quantitatively predicted
`such an effect
`from the reported bulk/cytoplasm partition coefficient
`(see the
`Discussion) on the assumption that the rates of growth and reproduction of the
`survivors is unaffected by the presence of the antimicrobial. We found, however, that
`the concentration of preservative (methyl and propyl parabens) in the bulk phase does
`not change detectably by analysis of the supernatant (UV spectrophotometry and high
`pressure liquid chromatography) after removing the bacteria by centrifugation from
`samples taken frequently over
`the entire course of the kill-minimum-regrowth
`sequence. Adaptation was confirmed as
`the causative mechanism by using the
`survivors of the regrowth process in 90% saturated propyl paraben as inoculum into a
`fresh propyl paraben solution;
`they grew out promptly while a naive inoculum
`reenacted the kill-minimum-regrowth sequence.
`In later experiments we foundthat the survivors of a single exposure to propyl paraben
`retained their immunity completely after forty one days of repeated culturing in the
`absence of the preservative; to this extent the adaptation is permanent and, as such,it
`may help explain why extraordinarily refractory strains are occasionally encountered in
`cosmetic manufacture.
`
`Butyl paraben at high saturation fraction in water initially kills E. col (but not
`Pseudomonas aeruginosa) much more rapidly than the lower esters. An inoculum of 10°
`to 10° appears to have been extinguished completely after only an hour or so of
`exposure to a 90% saturated solution and for several tens of hours no survivors are
`recovered but as with propyl paraben this may be followed by explosive regrowth. In
`this case, however, survivors transferred to fresh butyl paraben solution did not fare
`muchbetter than the naive culture. Because its performance was poor for practical
`purposes against E. coli and even poorer against other bacteria as reported in this paper,
`we did not pursue further the interesting matter of its distinctive, non-Ferguson
`behavior.
`
`Finally, we found benzyl paraben at near saturation in water so feebly antimicrobial
`even against S. awreus, that we omitted it from consideration as a useful preservative
`after only a few further trials.
`In this paper we report on some additional experiments in water and on more recent
`work in prototype products designed to simulate a wide range of real cosmetics.
`
`MATERIALS AND METHODS
`
`Both ATCC strains and wild isolates from products or processing equipment were
`used. The bacteria were grown at room temperature (ca. 23°C) for 48 hours in a
`nutrient-buffer salts-glucose solution, pH 6.7, adapted from that of Rye and Wiseman
`(8) shown in Table I. For convenience, it was prepared as a stock solution at twenty
`times the concentrations shown.
`
`The fungi were grown on Sabouraud Dextrose Agar (BBL) for seven days. The spores
`were harvested and suspended insaline.
`
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`78
`
`JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
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`Table I
`
`Nutrient-Buffer Solution, pH 6.7
`
`NH,Cl
`MgCl,
`Na,SO,
`Na,HPO,
`KH,PO,
`Glucose
`
`0.05M
`0.0005M
`0.00053M
`0.05M
`0.05M
`1g/l
`
`The compositions of the prototype products, a mineral oil emulsion, a vegetable oil
`emulsion and a shampoo,are given in Tables II, III and IV, They were prepared from
`ordinary cosmetic raw ingredients without special efforts to avoid contamination.
`Usually, a one-kilogram batch was prepared without preservative, withholding a few
`per cent of the water. The desired amount of preservative was weighed into a 100-g
`
`Table II
`Mineral Oil Emulsion
`
`Ingredient
`
`
`Per kg
`
`Light mineral oil
`Oleyl alcohol, 10 mole ethoxylate
`Nutrient-Buffer Stock Solution’
`Preservative
`Water
`20 times concentrations in Table I.
`
`200 g
`30 g
`5.0 ml
`q.s.
`to Lkg
`
`Table III
`Peanut Oil Emulsion
`
`Ingredient
`
`
`Per kg
`
`Peanut Oil (Planters’, 100%)
`Stearyl alcohol, 2 mole ethoxylate
`Stearic acid, 40 mole ethoxylate
`Nutrient-Buffer Stock Solution’
`Preservative
`Water
`"20 times concentrations in Table I.
`
`Shampoo
`
`Ingredient
`
`Table IV
`
`Sodium lauryl sulfate, 100%
`Sodium lauryl ether (2 mole) sulfate, 30%
`Lauroyl diethanolamide
`Linoleoyl diethanolamide
`Sodium chloride
`Oxthophosphoric acid, 85%
`Nutrient-Buffer Stock Solution’
`Preservative
`Water
`‘20 times concentrations in Table I.
`
`200 g
`1Sg
`20 g
`5.0 ml
`qs.
`tolkg
`
`Per kg
`
`75g
`100 g
`35g
`10 g
`2.0 g
`3.0 g
`5.0 ml
`qs.
`to Lkg
`
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`THE PARABENS
`
`79
`
`sub-batch in an eight-ounce screw-cap jar. The preservative was dissolved by heating
`for several hours at 60°C with occasional mixing. After cooling to room temperature,
`the pH was adjusted with 4N HCl or NaOHand water was added to 100.0 g.
`Emulsions prepared in this fashion are of poor stability but when higher levels of
`emulsifiers were used to improve the quality of the base formulas, the addition of each
`paraben hada specific degrading effect, in some cases causing phase inversion.Sinceit
`would have been pointless to compare,
`say, methyl and ethyl parabens in an
`oil-in-water system with propyl and butyl parabens in water-in-oil, we accepted
`uniformly poorstability as the lesser evil.
`The concentration (basis water content) of nutrient salts and glucose in the prototype
`products is about one-eighth of that in the aqueous broths. The intent here is to
`swamp out the possibly distorting effects of chance nutrification and the nutrient
`differences inherent in the three product formulas.
`It was not possible to measure inoculum growth in unpreserved control systems
`because these were invariably found grossly contaminated with stray microbes but the
`rapid growth to about 10’/g of recognizable inoculum bacteria and the persistence of
`mold spores in poorly preserved systems left no doubt that these prototype products,
`like their real cosmetic product counterparts will support damaging growth of the
`challenge organisms.
`Systems challenged with bacteria at 10°/g or mold spores at 10°/g were incubated at
`room temperature. Aliquots were diluted in one tenth strength Nutrient Broth (BBL),
`dispersed in Nutrient Agar (BBL) and incubated for three days at room temperature
`before counting. All challenged systems were sampled about one hourafter inocula-
`tion, on day 1, 2, 3 or 4 and on days, 7, 14 and 21. Sampling was terminated on orafter
`day 7 only if two successive counts clearly showed persistence or growth ofbacteria.
`
`RESULTS
`
`Water nutrified with mineral salts and glucose, buffered at pH 6.7 and saturated with
`methyl paraben successfully resisted challenge by two fungi and by thirteen gram-
`negative bacterial strains including the most resistant wild isolates in our collection. In
`the same medium saturated with ethyl paraben,five of the thirteen bacteria grew out;
`propyl paraben failed against ten of them and butyl parabenfailed against all but one
`bacterium and one mold. Table V shows these results in the form of the kill time
`which we define throughoutthis report as the earliest time in the sampling schedule at
`which the count of survivors was less than 10/g (no survivors detected) and remained
`so until 21 days after inoculation. These data clearly rank the parabens: methyl >
`ethyl > propyl > butyl. (Theyalso imply a ranking of the challenge organisms in terms
`of their ability to resist attack by the parabens, and they arelisted in Table V in this
`fashion.) Several of the entries in Table V are “G(A)” indicating growth after
`adaptation. In these cases 95% or more of the inoculum died in the first few days but
`the survivors grew to the limit of the nutrient system.
`In Table VI we show the kill time of P. aeruginosa ATCC #9721 in saturated aqueous
`paraben solutions at various pH’s. In this experiment
`there is less discrimination
`among the parabens, but
`the indication remains that
`the efficacy ranking is not
`strongly pH dependent; from low to high pH, methyl or ethyl paraben is the most
`potent, butyl paraben isleast.
`
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`80
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`JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
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`Table V
`Kill Time of Parabensat Saturation in Nutrient-Buffer Solution pH 6.7
`Kill time, days*
`
`Microbe
`Serratia marcescens
`Pseudomonas aeruginosa
`Pseudomonas aeruginosa
`Enterobacter hafnia
`Serratia liquifaciens
`Pseudomonas cepacia
`Pseudomonas aeruginosa
`Pseudomonas aeruginosa
`Serratia rubidaea
`Pseudomonas putida
`Enterobacter cloacae
`Escherichia colt
`Enterobacter hafnia
`Aspergillus niger
`Penicillium species
`
`Methyl
`Ethyl
`Propyl
`
`Origin
`Code
`Paraben
`Paraben
`Paraben
`Wild
`ED-2
`7
`G
`G
`Wild
`MEM
`1
`G
`G
`Wild
`BB-1A
`1
`G(A)
`G
`Wild
`LSC
`1
`G(A)}
`G
`Wild
`T-1
`1
`G(A)
`G
`Wild
`RS
`7
`7
`G
`Wild
`SM-5
`4
`14
`G
`ATCC
`#9721
`4
`14
`G
`Wild
`CW-1
`4
`4
`G
`Wild
`SM-6
`1
`1
`G
`Wild
`PLS-2
`1
`1
`1
`ATCC
`#25922
`1
`1
`1
`Wild
`SG
`4
`14
`14
`ATCC
`#16404
`4
`4
`4
`Wild
`7
`1
`7
`
`Butyl
`Paraben
`G
`G
`G
`G
`G(A)
`G
`G
`G
`G
`G
`G(A)
`G(A)
`14
`> 21
`1
`
`*G indicates heavy growth; G(A)indicates growth preceded by 95% orgreaterkill.
`
`Table VI
`Kill Time of Parabens at Saturation in Nutrient-Buffer Solution at
`Various pH’s Challenged with Pseudomonas aeruginosa ATCC #9721
`Kill time, days
`
`Methyl
`Ethyl
`Propyl
`Butyl
`
`pH
`Buffer’
`Paraben
`Paraben
`Paraben
`Paraben
`
`5.4
`6.7
`77
`86
`
`Malic acid
`Phosphate
`Tris-Phosphate
`Tris-Glycine’
`
`1
`1
`1
`1
`
`1
`G(A)
`1
`1
`
`1
`G(A)
`G(A)
`1
`
`G(A)
`G(A)
`G(A)
`1
`
`"Apart from the buffer changes and substitution of glycine for NH], the nutrients are as given in Table 1.
`“In this solution, glycine is also the source ofnitrogen.
`
`Table VII showsthe kill time of ED-2, a very resistant isolate identified as Serratia
`marcescens, in neutral mineral oil and peanut oil emulsions and in the shampoo, with
`and without nutrients with 0.8% nominal paraben level in all cases. In the mineral oil
`emulsion the methyl, ethyl and propyl parabens readily dissolve to this extent at 60°C
`but crystallize out in part on standing at room temperature; these systems are at
`saturation at about 0.6%. Re-precipitation does not occur with butyl paraben in the
`mineral oil emulsion nor with any of the parabens in the peanut oil emulsion or the
`shampoo;these systemsare at or below saturation.
`In the nutrified systems, only methyl paraben kills this organism in the emulsions; in
`the shampoo even methyl paraben fails to check its gtowth. In the absence of nutrient
`the preservatives do better in general; methyl and ethyl parabens are effective in the
`emulsions but propyl and butyl parabensstill fail, and in the shampooall four parabens
`fail.
`
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`

`THE PARABENS
`
`81
`
`Table VII
`Kill Time of 0.8% Paraben in Prototype Products at pH 6.5 Challenged with ED-2'
`
`
`
`
`
`Kill time, days
`
`Paraben No Nutrient*Medium Nutrified
`H
`Methyl’
`HR
`Ethyl?
`Propyl’
`Butyl
`
`aie
`
`DAAQAQOA!|ANAryaags
`
`Mineral Oil Emulsion
`
`QQee
`
`AAAA!;AAA
`
`Methyl
`Ethyl
`Propyl
`Butyl
`
`
`Peanut Oil Emulsion
`
`Shampoo
`
`‘Serratia marcescens, wild isolate.
`*Orchophosphate buffer.
`*Saturated.
`
`Methyl
`Ethyl
`Propyl
`Bucyl
`
`In the mineral oil emulsion at saturation the performance of the parabens is not very
`different from that in water. In Tables VII and IX we show kill time data on the first
`three parabens at saturation in the peanut oil emulsion and in the shampoo.
`Performance is marginally better in the peanut oil emulsion than in water, but the
`
`Kill Time of Parabens at Saturation in Nutrified Peanut Oil Emulsion, pH = 6.5
`
`Table VIII
`
`Microbe’
`
`
`A. niger, ATCC 16404
`P. aeruginosa, ATCC 9721
`EB-1
`ED-2
`
`Methyl
`Paraben
`1.0-1.2%
`
`2
`1
`1
`1
`
`Kill time, days”
`
`Ethyl
`Paraben
`0.8-1.0%
`
`2
`1
`2
`G
`
`Propyl
`Paraben
`1.2-1.6%
`
`7
`1
`1
`G
`
`'EB-1 and ED-2 are wild strains of Serratia marcescens.
`*Percentages afe approximate Concentrations.
`
`Table [X
`Kill Time of Parabens at Saturation in Nutrified Shampoo, pH 6.5 (ca. 2.5% in all cases)
`Kill time’
`
`
`Microbe’
`
`Methyl
`Paraben
`
`Ethyl
`Paraben
`
`Propyl
`Paraben
`
`1d
`1d
`1d
`A. niger, ATCC 16404
`th
`th
`th
`P. aeyuginosa, ATCC 9721
`G(A)
`ih
`th
`EB-1
`
`
`
`th thED-2 G(A)re
`‘EB-1 and ED-2 are wild strains of Serratia marcescens.
`*Days or hours as indicated.
`
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`82
`
`JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
`
`In the shampoo, kill rates are
`is still evident.
`ranking methyl > ethyl > propyl
`enhanced relative to saturated water, but propyl paraben, even at a concentration of
`about 2.5%, ultimately fails against two of the three bacteria.
`Binary mixtures of the parabens were examined in the emulsions as shownin Tables X
`and XI, which showkill times for P. aeruginosa and ED-2. In the peanut oil emulsion
`
`Table X
`Kill Time of Methyl Paraben and Mixtures in Nutrified Peanut Oil Emulsion,pH ca.6.7
`
`Paraben System
`
`0.8% methyl
`0.4% methyl
`0.4% methyl, 0.4% echyl
`0.4% methyl, 0.4% propyl
`0.4% methyl, 0.4% butyl
`
`"ATCC 9721
`
`Kill time, days
`
`P. aeruginosa’
`
`1
`G
`1
`G(A)
`G
`
`5
`
`E
`
`2
`G
`G
`G
`G
`
`Table XI
`Kill Time of Methyl Paraben and Mixtures in Nutrified Mineral Oil Emulsion, pH ca. 6.5
`Kill time, days
`
`
`
` Paraben System P. aeruginosa’ ED-2
`
`
`
`0.8% methyl?
`0.4% methyl
`0.4% methyl, 0.4% ethyl
`0.4% methyl, 0.4% propyl
`*ATCC 9721
`*Saturated.
`
`1
`1
`1
`1
`
`2
`G
`1
`G
`
`methyl paraben suffices at 0.8% but fails against both organisms at 0.4%. Addition of
`0.4% of a second paraben gives improvement in the order ethyl > propyl > butyl, but
`in no case is the more resistant bacterium killed asit is by 0.8% methyl paraben alone.
`The mineral oil system is similar except that the methyl/ethy! combination is a bit
`better than methy! alone. Note that this is not an equal weight comparison because of
`partial recrystallization of the methyl paraben at 0.8%. If we take the solubilities of both
`methyl and ethyl paraben as 0.6% in this system, then at 0.4% of each (two-thirds of
`saturation with each) then the cumulative saturation fraction is about 1.3. In aqueous
`broths we have found that such multiply saturated systems can be even morelethal
`than methyl paraben alone at saturation since the saturation scale extends beyond
`unity.
`
`DISCUSSION
`
`Lang and Rye(9) found that the growth ofE. co/f remains exponential or log-linear in
`the presence of methyl, ethyl and propyl parabens with decreasing slope up to about
`half their saturation concentrations. To a good approximation,
`their data can be
`summarized as a demonstration that the growth rate constant, &, in N = N°e®, depends
`on the paraben saturation fraction as
`
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`

`THE PARABENS
`
`k= k{1 — as,),
`
`83
`
`(I)
`
`where &, is the growth rate constant when no antimicrobials are present and s; is the
`saturation fraction of the ith paraben.
`The Ferguson principle is implied by the absence of the subscript on the dimensionless
`constant @ (which has a value of about 2.0); all three parabens have the same inhibitory
`effect when their levels are expressed as fraction of saturation.
`By independent radiochemical measurements, Lang and Rye also showed that the
`intracellular paraben concentration, ¢,
`is approximately the same for all
`three
`homologs whentheir equilibrium levels in the extracellular or bulk phase are expressed
`as Saturation fractions,5;:
`
`G=fig=f* c/o; =f* 5;
`
`(11)
`
`is the bulk concentration and @;is the solubility. The constant f* like the
`where ¢;
`constant @ in Equation I, has the same value for all three homologs (about 7.0 g/l).
`In the Lang and Rye study,
`the applicability of the Ferguson principle is both
`demonstrated and “explained,” where “explanation” follows from the plausible
`assumption that the parabens are equitoxic at equal intracellular concentrations. The
`assumption is plausible, in turn, on the further conjecture that the parabens are toxic to
`microbes because they partition reversibly into the lipid bilayer of the cell membrane
`and disorder its barrier function and the functions of embedded transport enzymes. A
`molecule of one homolog ought then to be aboutas disruptive as that of another.
`In retrospect,
`it
`is not
`too surprising that such a structure of assumptions and
`conjectures failed to support extrapolation. All that remains of the Ferguson principle
`in the range of paraben concentrations beyond half saturation (the limit of the Lang
`and Rye study) is an indication that at low levels of inoculation the initial kill rate is
`given by Equation I. Thereafter, survival and growth are determined by the rate of
`adaptation which increases markedly with the molecular weight of the paraben.
`Solubility in the medium does not serve as the sole index of efficiency as it would if
`the Ferguson principle were applicable, but
`it remains a crucial property. Methyl
`paraben is a potent antimicrobial in waterat saturation at 0.2%, butit fails at 0.4% in the
`emulsions and at 0.8% in the shampoo; it is a good preservative only for products in
`whichit is not too soluble. Propyl paraben is inadequate in water at 0.03% and remains
`so at 0.8% in the emulsions and even at about 2.5% in the shampoo.
`For practical purposes, our earlier solubility-efficacy proposal(1,2) is supplanted by a
`strong endorsement of methyl paraben as the best member ofthe series, to be used at
`the highest practical concentration, with a secondary recommendation of ethyl
`paraben as a supporting preservative when the amount of methyl paraben that can be
`usedis limited by regulation (0.4% maximum in Brazil, for example) or by solubility at
`low storage temperatures. Only rarely might it be useful to include propyl paraben as a
`third preservative.
`
`REFERENCES
`
`(1) J. J. O'Neill, P. L. Peelor, A. F. Peterson and C. H. Strube, Application of the Ferguson principle to the
`selection of sparingly soluble preservatives, Devel. Indust. Microbiol., 19, 335-345 (1978).
`(2) J. J. O'Neill, P. L. Peelor, A. F. Peterson and C. H. Strube, Selection of parabens as preservatives for
`cosmetics and toiletries,J. Soc. Cosmet. Chem., 30, 25-38 (1979).
`
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`84
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`JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
`
`(3) W. P. Evans, Applicability of the Ferguson principle to systems of mixed preservatives, J. Pharm.
`Pharmacol., 17, 217-221 (1965).
`(4) J. Ferguson, The use of chemical potentials as indices of toxicity, Proc. Roy. Soc. Lond., Sect B, 127,
`387-404 (1939).
`(5) N. A. Allawala and S, Riegelman, Phenol coefficients and the Ferguson principle, J. Am. Pharm.
`Assoc., Sci. Ed., 43, 93-97 (1954).
`(6) R. T. Gottlesman and D.Chin,Salicyclic acid and related compoundsin Kirk-Othmer Encyclopedia of
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`;
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`ethyl and propyl p-hydroxybenzoates, J. Pharm. Pharmac., 24, 160P-161P (1972).
`
`UCB Biopharma SPRL(IPR2019-00400)
`Exhibit 2015 Page 10
`
`UCB Biopharma SPRL (IPR2019-00400)
`Exhibit 2015 Page 10
`
`