Msn Laboratories Private Ltd., Msn Pharmaceuticals, Inc. v. Bausch Health Ireland Ltd.

NOTE: This disposition is nonprecedential.
United States Court of Appeals
for the Federal Circuit
______________________
MSN LABORATORIES PRIVATE LTD., MSN
PHARMACEUTICALS, INC.,
Appellants
v.
BAUSCH HEALTH IRELAND LTD.,
Appellee
______________________
2024-1053
______________________
Appeal from the United States Patent and Trademark
Office, Patent Trial and Appeal Board in Nos. IPR2022-
00722, IPR2023-00016.
______________________
Decided: June 20, 2025
______________________
TUNG -O N K ONG , Wilson, Sonsini, Goodrich & Rosati,
PC, San Francisco, CA, argued for appellants. Also repre-
sented by RICHARD J. BERMAN , JANINE A. CARLAN ,
BRADFORD C. FRESE ArentFox Schiff LLP, Washington,
DC.
JUSTIN JAMES H ASFORD , Finnegan, Henderson,
Farabow, Garrett & Dunner, LLP, Washington, DC,
Case: 24-1053 Document: 13 Page: 1 Filed: 06/20/2025

-- 1 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
2
argued for appellee. Also represented by BRYAN D INER,
JOSHUA G OLDBERG .
______________________
Before LOURIE, D YK , and CHEN , Circuit Judges.
LOURIE, Circuit Judge.
MSN Laboratories Private Ltd. and MSN Pharmaceu-
ticals, Inc. (“MSN”) appeal from a final written decision of
the United States Patent and Trademark Office Patent
Trial and Appeal Board (“the Board”) holding that it had
not shown claims 1–6 of U.S. Patent 7,041,786 (“the ’786
patent”) to be unpatentable as obvious. Mylan Pharms.,
Inc., v. Bausch Health Ireland Ltd., No. IPR2022-
00722, 2023 WL 6161595 (P.T.A.B. Sept. 8, 2023) (“Deci-
sion”). For the reasons provided below, we vacate and re-
mand.
BACKGROUND
Bausch Health Ireland Ltd. (“Bausch”) owns the ’786
patent, which claims a compound that is part of a group of
structurally related peptides known as “guanylate cyclase
receptor agonists.” ’786 patent col. 1 ll. 14–15. Guanylate
cyclase receptor agonists play an important role in the op-
eration of the GI tract. By binding to receptors, they stim-
ulate the production of cyclic guanosine monophosphate
(“cGMP”), which in turn activates a complex signaling
pathway that regulates sodium and water secretion in the
intestinal lumen. ’786 patent col. 1 ll. 34–35. Guanylate
cyclase receptor agonists can be administered in various
formulations, such as “solutions, powders, suspensions,
emulsions, tablets, capsules, transdermal patches, [or]
ointments.” ’786 patent col. 13 ll. 24–27. Some guanylate
cyclase receptor agonists exhibit what is known as “topoi-
somerism,” meaning that an individual peptide can exist in
different three-dimensional structures with different levels
of biological activity. Decision at *8–9; J.A. 04279.
Case: 24-1053 Document: 13 Page: 2 Filed: 06/20/2025

-- 2 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
3
One such agonist in the prior art is uroguanylin, which
is known to have two topoisomers: an active “A form” which
can bind to receptors and stimulate production of cGMP,
and an inactive “B form,” which cannot. Decision at *8– 9;
J.A. 03404. Obviousness of what is claimed in the ’786 pa-
tent over uroguanylin is at issue in this appeal.
Claim 1 of the ’786 patent, which is exemplary, reads
as follows:
1. A peptide consisting of the amino acid of SEQ
ID NO:20.
’786 patent, col. 37 ll. 2–3. SEQ ID NO:20 is the amino acid
sequence for plecanatide, a synthetic analog of uroguan-
ylin. Id. at col. 35; Decision at *3. The sole difference be-
tween uroguanylin and plecanatide, as shown below, is a
single substitution at the third position: in plecanatide, as-
partic acid (Asp) is replaced with glutamic acid (Glu).
Uroguanylin: Asn1 -Asp 2 -Asp3-Cys 4 -Glu5 -Leu6 -
Cys 7 -Val8 -Asn9 -Val10 -Ala11 -Cys 12 -Thr 13 -Gly14 -
Cys 15 -Leu16
Plecanatide: Asn1 -Asp 2 -Glu3-Cys 4 -Glu5 -Leu6 -
Cys 7 -Val8 -Asn9 -Val10 -Ala11 -Cys 12 -Thr 13 -Gly14 -
Cys 15 -Leu16
See Decision at *2. Glutamic acid and aspartic acid are
chemically similar—the only difference is that glutamic
acid has a second methylene moiety in its side chain. They
are also the only two natural amino acids negatively
charged at a neutral pH. Bausch markets and sells pleca-
natide under the brand name Trulance® for use as an oral
laxative to treat constipation.
In March 2021, MSN and Mylan Pharmaceuticals Inc.,
(“Mylan”) separately petitioned for inter partes review, as-
serting that all claims of the ’786 patent would have been
obvious over U.S. Patent 5,489,670 (“Currie”), an article
entitled “Purification, cDNA Sequence, and Tissue
Case: 24-1053 Document: 13 Page: 3 Filed: 06/20/2025

-- 3 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
4
Distribution of Rat Uroguanylin” (“Li”), and other second-
ary references. Regarding claim 1, MSN and Mylan con-
tended that a person of ordinary skill in the art would have
been motivated to substitute aspartic acid with glutamic
acid at uroguanylin’s third position to arrive at plecanatide
based on: (1) Currie’s disclosure that uroguanylin
“may . . . act as a laxative and be useful in patients suffer-
ing from constipation,” J.A. 00237 (citing J.A. 01897), (2)
Li’s disclosure that the substitution would create a com-
pound with affinity to guanylate cyclase receptors “compa-
rable” to that of uroguanylin, J.A. 00237
(citing J.A. 01910), and (3) evidence that the substitution
was “conservative.” J.A. 00236–37 (citing J.A. 01347–48).
In its preliminary response, Bausch made two main ar-
guments why MSN and Mylan had not established a rea-
sonable likelihood that claim 1 would be shown to have
been obvious. Mylan Pharms., Inc., v. Bausch Health Ire-
land Ltd., No. IPR2022-00722, Paper No. 6, at 39–62
(P.T.A.B. Jun. 29, 2022). First, Bausch contended that a
person of ordinary skill in the art would not have been mo-
tivated to select uroguanylin as a lead compound for modi-
fication. Second, Bausch argued that a person of ordinary
skill in the art would not have been motivated to substitute
aspartic acid with glutamic acid at uroguanylin’s third po-
sition with a reasonable expectation of success.
The Board disagreed, and after joining the two peti-
tions, granted institution. During the trial phase of the
proceedings, Bausch reiterated its two prior arguments
and also made a new contention. Specifically, Bausch ar-
gued that, even if there had been a motivation to select
uroguanylin as a lead compound and substitute aspartic
acid with glutamic acid at its third position, any case of ob-
viousness was outweighed by plecanatide’s unexpected re-
sults. J.A. 01114–23. In support of that additional
argument, Bausch pointed to several experiments compar-
ing properties of plecanatide and uroguanylin. Id. at
01115–23.
Case: 24-1053 Document: 13 Page: 4 Filed: 06/20/2025

-- 4 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
5
In its final written decision, the Board maintained that
a person of ordinary skill in the art would have been moti-
vated to select uroguanylin as a lead compound and substi-
tute aspartic acid with glutamic acid at its third position.
Decision at *7–14. Nevertheless, the Board held that MSN
and Mylan had not established that the challenged claims
would have been obvious, agreeing with Bausch that the
experiments it proffered demonstrated plecanatide’s unex-
pected results which were sufficient to outweigh the prima
facie case of obviousness. Id. at *15–24. In so concluding,
the Board found that Bausch’s experiments “reflect the use
of human uroguanylin as it naturally exists, i.e., a mixture
with some interconversion between topoisomers, which is
the closest prior art to the peptide of claim 1.” Id. at *16.
MSN and Mylan separately appealed and we consolidated
the cases for briefing and oral argument. Mylan has since
voluntarily dismissed its appeal, leaving only MSN’s pend-
ing. We have jurisdiction under 28 U.S.C. § 1295(a)(4)(A).
D ISCUSSION
We review the Board’s decisions under the Administra-
tive Procedure Act (“APA”). Taking “due account . . . of the
rule of prejudicial error,” we must hold unlawful and set
aside agency action, findings, and conclusions found to
be . . . arbitrary, capricious, an abuse or otherwise not in
accordance with the law” or “unsupported by substantial
evidence.” 5 U.S.C. § 706(2)(A), (E). “Substantial evi-
dence . . . means such relevant evidence as a reasonable
mind might accept as adequate to support a conclusion.”
Consol. Edison Co. of N.Y. v. NLRB, 305 U.S. 197, 217
(1938). In applying those standards, “we will uphold a de-
cision of less than ideal clarity if the agency’s path may rea-
sonably be discerned,” but “we may not supply a reasoned
basis for the agency’s action that the agency itself has not
given.” Rovalma, S.A. v. Bohler-Edelstahl GmbH & Co.
KG, 856 F.3d 1019, 1024 (Fed. Cir. 2017) (citations omit-
ted). Accordingly, “the Board must, as to issues made ma-
terial by the governing law, set forth a sufficiently detailed
Case: 24-1053 Document: 13 Page: 5 Filed: 06/20/2025

-- 5 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
6
explanation of its determinations both to enable meaning-
ful judicial review and to prevent judicial intrusion on
agency authority.” Id.
On appeal, MSN argues that the Board erred in con-
cluding that the experiments proffered by Bausch sup-
ported its argument of unexpected results. For its part,
Bausch argues that the Board’s conclusion as to unex-
pected results was supported by substantial evidence. Al-
ternatively, Bausch argues that the Board’s decision
should be affirmed because its threshold finding that a per-
son of ordinary skill in the art would have been motivated
to select uroguanylin as a lead compound for modification
was not supported by substantial evidence. We begin with
Bausch’s alternative affirmance argument.
I
Whether a new chemical compound would have been
prima facie obvious depends in part on a comparison with
its closest prior art. See e.g., In re Payne, 606 F.2d 303,
315– 16 (CCPA 1979) (collecting cases). In doing so, a court
may utilize what has become known as a “lead compound”
analysis, which ordinarily follows a two-part inquiry.
Otsuka Pharm. Co. v. Sandoz, Inc., 678 F.3d 1280, 1291
(Fed. Cir. 2012). In such a case, the first inquiry is whether
a person of ordinary skill in the art “would have selected
the asserted prior art compound[] as [a] lead compound[],
[i.e.,] . . . a compound in the prior art that would be most
promising to modify in order to improve upon its activity
and obtain a compound with better activity.” Id. (citation
omitted). The second inquiry “is whether the prior art
would have supplied one of ordinary skill in the art with a
reason or motivation to modify a lead compound to make
the claimed compound with a reasonable expectation of
success.” Id. at 1292 (citations omitted).
The Board concluded that MSN had established a
prima facie case of obviousness, first determining that a
person of ordinary skill in the art would have recognized
Case: 24-1053 Document: 13 Page: 6 Filed: 06/20/2025

-- 6 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
7
uroguanylin as a good candidate for treating constipation
and therefore selected it as a lead compound for further
modification. See Decision at *7–9. That finding was sup-
ported by substantial evidence. As noted, Currie discloses
that uroguanylin “may . . . act as a laxative and be useful
in patients suffering from constipation.” J.A. 01897. And
although uroguanylin was known to interconvert between
its active A form and inactive B form, particularly at low
pH levels, thereby decreasing its affinity for guanylate
cyclase receptors, MSN’s expert testified that that instabil-
ity could be mitigated using simple techniques known to a
person of ordinary skill in the art. Decision at *9 (cit-
ing J.A. 03200 ¶ 29 (MSN expert declaration stating that a
person of ordinary skill in the art could easily formulate a
tablet of uroguanylin with a protective coating to target re-
lease in the higher pH environment of the small intestine
as opposed to the lower pH environment of the stomach)).
It was therefore reasonable for the Board to conclude that
uroguanylin was a promising compound to modify to im-
prove upon its already-known laxative effects.
Bausch contends that the Board’s conclusion lacks sub-
stantial evidence support because another guanylate
cyclase receptor agonist, bacterial ST peptide, was “a far
more promising option” than uroguanylin for treating con-
stipation. Resp. Br. 68. Specifically, Bausch argues that
because bacterial ST peptide was “not afflicted with topoi-
somerism” and produced “remarkabl[y]” more cGMP than
uroguanylin, a person of ordinary skill in the art would
have been motivated to select bacterial ST peptide as a lead
compound instead of uroguanylin. Id. at 68–69. But the
Board already considered and reasonably rejected that ar-
gument. It explained that, although uroguanylin was in
those respects “disadvantag[ous] relative to [bacterial ST
peptide],” a person of ordinary skill in the art would still
select uroguanylin over bacterial ST peptide as a lead com-
pound because bacterial ST peptide was derived from path-
ogenic bacteria, and thus “dangerous” for administration in
Case: 24-1053 Document: 13 Page: 7 Filed: 06/20/2025

-- 7 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
8
humans. Decision at *8–9. That finding is consistent with
Currie and Li. J.A. 01897 (Currie: bacterial ST peptide is
known to be a “major cause” of diarrhea, causing dehydra-
tion); J.A. 01901 (Li: exposure to high levels of bacterial ST
peptide “produces a watery diarrhea that can lead to dehy-
dration and death”). Accordingly, the Board’s conclusion
that a person of ordinary skill in the art would have se-
lected uroguanylin as the most promising compound to im-
prove upon its effects as a laxative was supported by
substantial evidence.
In view of the foregoing, and because Bausch does not
challenge the Board’s determination that a person of ordi-
nary skill would have been further motivated to modify the
third position of uroguanylin to make plecanatide with a
reasonable expectation of success, we do not disturb the
Board’s determination that MSN established a prima facie
case of obviousness. We accordingly move to the issue of
unexpected results.
II
One way to rebut a prima facie case of obviousness is
to demonstrate “unexpected results.” E.g., In re Harris,
409 F.3d 1339, 1343 (Fed. Cir. 2005). In the context of
chemical compounds, such a showing is often made
through comparative testing, i.e., by demonstrating
through experimentation that the “claimed compound[]
possess[es] unexpectedly advantageous or superior proper-
ties” relative to a lead compound. In re Payne,
606 F.2d 303, 315–16 (CCPA 1979).
As noted, Bausch proffered several experiments com-
paring the chemical properties of plecanatide, uroguanylin,
and other guanylate cyclase receptor agonists in support of
its argument that substituting aspartic acid for glutamic
acid at uroguanylin’s third position produced unexpected
results. Specifically, the experiments compared: (1) the
amount of cGMP produced by each peptide at a neutral pH,
(2) the amount of cGMP produced by each peptide across
Case: 24-1053 Document: 13 Page: 8 Filed: 06/20/2025

-- 8 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
9
different pHs found in the GI tract, (3) the binding affinity
of each peptide for guanylate cyclase receptors, and (4) the
ratio of active to inactive topoisomers after each peptide
was incubated in an acidic solution.1 Based on the results
of those experiments, the Board found that plecanatide ex-
hibited unexpectedly increased potency, pH sensitivity,
binding affinity, and topoisomeric stability relative to
uroguanylin, and that those unexpected results were suffi-
cient to outweigh the prima facie case of obviousness. De-
cision at *15–24.
MSN argues that the Board legally erred by relying on
those experiments. First, MSN argues that the experi-
ments did not compare the claimed plecanatide with the
“closest prior art.” Open. Br. 26–31. Second, MSN asserts
that the experiments did not comport with the requirement
that unexpected results must be “commensurate in scope”
with the challenged claims. Id. at 31–36. Third, MSN con-
tends that the experiments were insufficient to support a
finding of non-obviousness because they merely demon-
strate “differences in degree,” not the required “differences
in kind.” Id. at 36–44. And fourth, MSN argues that the
experiments did not undermine the prima facie case of ob-
viousness. Id. at 44–53. We need only address the first ar-
gument to resolve this appeal.
To demonstrate that unexpected results are attributa-
ble to a claimed compound as opposed to some other non-
claimed factor, a patent owner “relying upon a comparative
showing to rebut a prima face case must compare [the]
claimed [compound] with the closest prior art”—i.e., the
most similar compound found in the prior art. In re
1 Bausch also proffered an experiment that it con-
tended demonstrated that plecanatide was unexpectedly
more heat stable than uroguanylin. The Board disagreed
and Bausch does not appeal that finding. Decision at *21–
22.
Case: 24-1053 Document: 13 Page: 9 Filed: 06/20/2025

-- 9 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
10
Merchant, 575 F.2d 865, 869 (CCPA 1978) (collecting
cases). Thus, where one or both of the claimed or lead com-
pounds can exist in different three-dimensional structures,
it follows that for comparative testing to satisfy the closest-
prior-art requirement, it must compare the “structurally
closest” forms of those compounds. See Astrazeneca
Pharms. LP v. Teva Pharms. USA, Inc., 583 F.3d 766, 775
(Fed. Cir. 2009) (citing Merchant, 575 F.3d at 868)).
At the Board, MSN’s argument that the experiments
did not compare the claimed plecanatide with the closest
prior art was built upon multiple premises. First, MSN as-
serted that plecanatide, like uroguanylin, exhibits topoi-
somerism and could exist in both an active A form and
inactive B form. See J.A. 00928–29. And because the chal-
lenged claims did not recite any structural limitations,
MSN asserted that they encompassed both the active A and
inactive B forms of plecanatide. Id. In support of its posi-
tion, MSN proffered submissions made by Bausch to the
European Patent Office (“EPO”) stating that plecanatide
“can exist in two different iso[mers], only one of which is
biologically active.” Id. (citing J.A. 03352). Furthermore,
MSN contended that the experiments only used the active
A form of plecanatide. J.A. 00928 (citing J.A. 01371–
72 ¶ 168)).
Next, MSN contended that highly pure active A-form
uroguanylin could have been used for comparative testing.
In support of this contention, MSN presented additional re-
search showing that the highly pure active A form of
uroguanylin had been shown to be easily purified and
maintained in acidic and slightly alkaline solutions and as
a lyophilized powder. Id. (citing J.A. 04286 Fig 6C: 1% in-
terconversion of A- to B-form uroguanylin after one hour at
pH 4.5 and 5% interconversion after 70 hours at pH 7.7,
J.A. 04292 lyophilized powder of A-form uroguanylin re-
mained 99% topoisomerically pure after one year in stor-
age).
Case: 24-1053 Document: 13 Page: 10 Filed: 06/20/2025

-- 10 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
11
MSN therefore asserted that because Bausch’s experi-
ments compared the active A form of plecanatide with a
mixture of active A- and inactive B-form uroguanylin, when
similarly pure active A-form uroguanylin could have been
used, the requirement that comparative testing must be
between the claimed compound and the structurally closest
lead compound was not satisfied. J.A. 00929.
The Board disagreed. It explained, relying on addi-
tional research presented by Bausch showing that “topolog-
ical isomers of human uroguanylin are not stable in
solution and are readily interconvertible,” that “[s]ome
amount of interconversion to the inactive conformation was
inevitable in [] testing.” Decision at *16 (citing J.A. 04291).
Accordingly, the Board found that the closest prior art to
the claimed plecanatide was not highly pure active A-form
uroguanylin, but rather a “mixture” of A- and B-form
uroguanylin. Id. It then found that such a mixture was
used in the experiments. Decision at *16 (“Patent Owner’s
tests reflect the use of human uroguanylin as it naturally
exists, i.e., a mixture with some interconversion between
topoisomers.”) (citing no evidence in support). The Board
did not come to a conclusion as to whether plecanatide ex-
hibits topoisomerism. Id. (“[T]he extent to which [pleca-
natide] interconverts between an active and inactive
topoisomer, if at all, is not clear on this record.”). Nor did
it address the statements put forward by MSN that Bausch
made at the EPO suggesting that plecanatide can exist in
both active and inactive topoisomers. Id. Those would
seem to be essential inquiries.
We therefore conclude that the Board’s determination
that the experiments compared the claimed invention with
the closest prior art lacks adequate explanation, for three
reasons. First, the Board sidestepped the critical threshold
issue of whether the claims encompass plecanatide com-
pounds in both the active and inactive forms. As noted, to
satisfy the closest-prior-art requirement in this context,
the patent owner must compare the “structurally closest”
Case: 24-1053 Document: 13 Page: 11 Filed: 06/20/2025

-- 11 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
12
forms of the claimed compound and lead compound. See
Astrazeneca, 583 F.3d at 775 (citations omitted). Nor-
mally, that requires determining the structural character-
istics of what is said to be the claimed compound. See id.
But the Board did not do so here, explicitly leaving open
the question whether the challenged claims encompass
both active and inactive topoisomers of plecanatide. Deci-
sion at *16. We therefore will vacate and remand the
Board’s decision that testing the claimed compound
showed unexpected properties over the prior art. On re-
mand, the Board should determine, based on the evidence
put forward by the parties, whether plecanatide exhibits
topoisomerism and whether the testing of the challenged
claims should encompass both the active and inactive
forms.
Second, the Board did not address MSN’s contention
that highly pure A-form uroguanylin could be maintained
as a lyophilized powder. Guanylate cyclase receptor ago-
nists can be administered in both solution and powder for-
mulations. ’786 patent col. 13 ll. 24–27. If it is true, as
MSN contends, that uroguanylin can be maintained and
therefore tested in its active A form as a lyophilized pow-
der, that would support MSN’s argument that the experi-
ments did not use the closest prior art. But in finding that
the closest prior art is a “mixture” of A- and B-form
uroguanylin, the Board only cited evidence stating that
uroguanylin solutions could not be maintained for ex-
tended periods of time. Decision at *16 (cit-
ing J.A. 004291). Its broad conclusion that a mixture of A-
and B-form uroguanylin—regardless of its formulation—is
the closest prior art therefore does not reasonably follow
from the solution-specific evidence it cited in support.
Finally, the Board did not address whether the experi-
ments submitted by Bausch actually used what it deter-
mined to be the closest prior art—a solution consisting of a
mixture of A- and B-form uroguanylin. Of the four experi-
ments, only two (analyzing cGMP production and pH
Case: 24-1053 Document: 13 Page: 12 Filed: 06/20/2025

-- 12 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
13
sensitivity) provide any information regarding the precise
composition of the peptide solutions used, stating that the
peptides were “purified (>95% purity) using a published
procedure (38).” ’786 patent, col. 15 l. 55. But purity in
this context can refer to either of two different things: pu-
rifying uroguanylin topoisomers from one another or puri-
fying uroguanylin from other compounds. Published
procedure 38 (“Klodt”), does not appear to specify what
level of purity the procedure it used is referring to. See
J.A. 04696–04704. As for the other two experiments, nei-
ther indicates whether the peptides used were purified at
the topoisomer or compound level. J.A. 05372 (binding af-
finity experiment: broadly stating without explanation
that the peptides analyzed “were purified by RP-HPLC”
prior to testing); J.A. 05070–87 (interconversion experi-
ment: providing no information as to how the analyzed pep-
tides were initially purified). On remand, the Board will
need to reevaluate the experimental evidence consistent
with its determination regarding what is the closest prior
art compound as discussed above.
To complicate this last issue further, MSN takes con-
tradictory positions as to what level of purity Klodt refers.
In the context of its closest-prior-art argument, MSN as-
serts that Klodt refers to purity with respect to other com-
pounds, not topoisomerism. See Oral Arg. at 3:56–4:50,
available at https://oralarguments.cafc.uscourts.gov/de-
fault.aspx?fl=24-1011_04072025.mp3 (Klodt “doesn’t say
topoisomeric purity, it just says purity, [which refers to] ex-
cluding other peptides, [not] isolating a particular topoiso-
mer of a single peptide”); see also Reply Br. 5–6 (“The
Board never accepted [that] ≥95%” purity meant the
uroguanylin was “≥95% topoisomerically-pure A-form.”)
(emphasis in original). But for its commensurate-in-scope
argument, MSN asserts that the same procedure in Klodt
was used to purify peptides at the topoisomer level. See
Open. Br. 31 (stating that because plecanatide was purified
using the procedure outlined by Klodt, only “purified A-
Case: 24-1053 Document: 13 Page: 13 Filed: 06/20/2025

-- 13 of 14 --

MSN LABORATORIES PRIVATE LTD . v.
BAUSCH HEALTH IRELAND LTD .
14
form” plecanatide was used in the cGMP production and
pH sensitivity experiments).
In sum, without more explanation from the Board as to
its determination that the experiments compared the chal-
lenged claims to the closest prior art, we cannot reach a
judgment on MSN’s challenges to the Board’s holding of
non-obviousness. Should the Board determine that the ex-
periments cannot be shown to have compared the closest
prior art compound, their value with respect to its unex-
pected results determination is considerably diminished.
CONCLUSION
We have considered the parties remaining arguments
and find them unpersuasive. For the foregoing reasons, we
vacate the Board’s decision and remand for further pro-
ceedings consistent with this opinion.
VACATED AND REMANDED
COSTS
The parties shall bear their own costs.
Case: 24-1053 Document: 13 Page: 14 Filed: 06/20/2025

-- 14 of 14 --