Monte Rosa Therapeutics : AACR 2024 – The GSPT1 Molecular Glue Degrader MRT-2359 Is Active against Prostate Cancer

#3294: The GSPT1 molecular glue degrader MRT-2359 is active against prostate cancer

Ralph Tiedt1, Martin Schillo1, Arnaud Osmont1, Débora Bonenfant1, Rajiv Narayan2, Owen Wallace2, Markus Warmuth2

1Monte Rosa Therapeutics AG, WKL-136.3, Klybeckstrasse 191, 4057 Basel, Switzerland

2Monte Rosa Therapeutics Inc., 321 Harrison Ave, Boston, MA 02118, United States

American Association for Cancer Research (AACR) Annual Meeting, April 5-10, 2024, San Diego

Introduction

Activity of MRT-2359 in an AR(-V7) positive / c-MYC high prostate cancer model

MRT-2359 + enzalutamide in an AR positive (V7 low) / c-MYC high model

We had previously observed preferential activity of GSPT1 molecular glue degraders (MGD) in MYC-driven breast and lung cancer cell models in vitro and in vivo1,2. The schematic depicted below outlines a mechanistic hypothesis that could explain this observation.

22RV1 prostate cancer cells express high levels of both full- length AR and the AR variant AR-V7 that lacks the ligand binding domain. 22RV1 cells were treated with MRT-2359 in vitro and protein levels of GSPT1, c-MYC and AR were

1	6 h	24 h
MRT-2359; µM	0 0.01 0.1 1 10 30	0 0.01 0.1 1 10 30
GSPT1
c-MYC

VCaP prostate cancer cells express mostly full-length AR and only minimal amounts of AR-V7. VCaP cells were treated with MRT-2359 or enzalutamide in vitro and protein levels of GSPT1, c-MYC and AR were measured (1). Subsequently, subcutaneous VCaP xenografts in immunodeficient castrated mice were treated with MRT-2359, enzalutamide, or a combination thereof (2).

MYC

DNA

Genes involved in protein synthesis e.g., eIF4E, 4EBP1 and 4EBP2

1

Addiction

To sustain growth, MYC-driven tumors are addicted to protein translation

measured (1). Subsequently, subcutaneous 22RV1 xenografts in immunodeficient, castrated mice were treated with MRT-2359, enzalutamide, or a combination thereof (2). Efficacy parameters were calculated on day 28 (3).

AR

AR-V7

GAPDH

1	enzalutamide; µM			MRT-2359; µM
0	0.1	1	10	30	0	0.001	0.01	0.1	1	3

GSPT1

c-MYC

2		VCaP AR positive (V7 low) / c-MYC high
		Vehicle
SEM)		MRT-2359, 3 mg/kg QD
800	MRT-2359, 10 mg/kg 5 days on / 9 days off
enzalutamide, 30 mg/kg QD
±		MRT-2359, 3 mg/kg QD + enzalutamide

• 4EBP1
  eIF4E

eIF4E complex

eIF4E

	P	P
mTOR	P	P
4EBP1

Ribosome with	3
growing
peptide chain	GSPT1
	eRF1

2

Dependency

Protein

This addiction creates a

dependency on the translation

termination factor GSPT1

	3
	Therapeutic vulnerability
AAAAA		GSPT1 is a therapeutic

2	2000	22RV1	no further treatment	3
SEM)	AR positive
	c-MYC high		Vehicle
±	1500			MRT-2359, 10 mg/kg QD
mean
		MRT-2359, 3 mg/kg 5 days on / 9 days off
			MRT-2359, 10 mg/kg 5 days on / 9 days off
,				MRT-2359, 30 mg/kg 5 days on / 9 days off
3
(mm	1000
		enzalutamide, 30 mg/kg QD
			MRT-2359, 10 mg/kg QD + enzalutamide
Tumor volume
500			MRT-2359, 10 mg/kg 5/9 + enzalutamide
0

Group	Treatment regimen	Day 0 TV	Day 28 TV	Change in TV	T/C or regression
(mm3, mean)	(mm3, mean)	(mm3)	(%)
1	Vehicle	189.5	1392.9	1203.4	100.0
2	MRT-2359, 10 mg/kg QD	189.5	0.0	-189.5	-100
3	MRT-2359, 3 mg/kg 5 days	189.7	422.9	233.2	20
on/9days off
4	MRT-2359, 10 mg/kg 5 days	189.4	42.3	-147.1	-78
on/9days off
5	MRT-2359, 30 mg/kg 5 days	189.5	7.6	-181.9	-96
on/9days off
6	Enzalutamide, 30 mg/kg QD	189.6	1125.8	936.2	78
7	MRT-2359, 10 mg/kg QD +	189.5	0.0	-189.5	-100
enzalutamide, 30 mg/kg QD

AR

AR-V7

GAPDH

In vitro treatment (panel 1): 24h. VCaP efficacy parameters (panel 2) on day 31: enzalutamide, 30 mg/kg QD, 27% T/C; MRT-2359,3 mg/kg QD, -32%regression; MRT-2359,10 mg/kg 5 days on / 9 days off, -8%regression; MRT-2359,3 mg/kg QD + enzalutamide, 30 mg/kg QD, -100%regression; MRT-2359,10 mg/kg 5 on / 9 off + enzalutamide, 30 mg/kg QD, -62%regression. Abbreviations: as before.

,mean	600	MRT-2359, 10 mg/kg 5 on / 9 off + enzalutamide
3
(mm	400
volume	200			TV = 0 in
Tumor				8/8 mice
0
	0	10	20	30

Time post treatment initiation (days)

mRNA

Initiation

	STOP
2	Termination

vulnerability

of MYC-driven tumors

leading to preferential activity of

GSPT1 MGDs

0

20

40

60

Time post treatment initiation (days)	TV = 0 in 10/10 mice treated with
MRT-2359 at 10 mg/kg QD

	MRT-2359, 10 mg/kg 5 days
8	on/9days off + enzalutamide,	189.6	44.7	-144.9	-76
	30 mg/kg QD

MRT-2359-mediated GSPT1 degradation led to marked reduction of c-MYC and AR after 24h in vitro. In vivo, MRT-2359 was efficacious upon continuous daily (QD) or intermittent (5 days on / 9 days off) administration. Combination with enzalutamide led to deeper regression. Daily administrations of 3 mg/kg MRT-2359 + 30 mg/kg enzalutamide caused full tumor regression (TV = 0) in all mice after 31 days of treatment.

Abbreviations: 5 days on / 9 days off, 5 daily doses of MRT-2359 followed by 9 days of drug holiday; AR, androgen receptor; QD, daily (quaque die); regression, % of tumor volume change relative to initial tumor volume of same group on day 0 (only calculated if tumors were shrinking on average); SEM, standard error of the mean; T/C, treated divided by control, relative % tumor volume change of treatment to vehicle group (only calculated if no regression); TV, tumor volume.

Opportunity for GSPT1 MGD in prostate cancer

MRT-2359 mediated GSPT1 degradation led to marked reduction of c-MYC and AR / AR-V7 after 24h in vitro. In vivo, MRT-

Higher expression of translation-related genes in sensitive prostate cell lines

MRT-2359 is a GSPT1 MGD, which was optimized to achieve preferential antiproliferative activity in MYC-driven lung cancer. While profiling MRT-2359 across hundreds of cancer cell lines representing multiple cancer types, we noted a clear segregation of prostate cancer cell lines into sensitive and resistant, associated with androgen receptor (AR) and MYC expression as potential biomarkers.

2359 was efficacious upon continuous daily (QD) or intermittent (5 days on / 9 days off) administration. MRT-2359 at 10 mg/kg QD led to full tumor regression (TV = 0), and there was no sign of tumor regrowth 35 days after treatment cessation. As expected, enzalutamide was not active in this model, and it did not contribute to efficacy in combination with MRT-2359.

GSEA (sensitive vs resistant)

Rank	Gene set	Size	NES
1	HALLMARK_ANDROGEN_RESPONSE	96	2.64

• Besides androgen receptor

expression and activity, sensitive

prostate cancer cell lines are

characterized by high expression of

1

% growth relative to DMSO

150

125

100

75

50

25

0
1	1

VCaP 22RV1

LNCaP (clone FGC) MDAPCa2b

PC3

DU145

NCIH660 LASCPC01**

* engineered N-MYC driven cell line,

2

adjusted P-value(-log10)

KLK4

3

AR

FOLH1

2

MYC KLK3

1	TMPRSS2

Activity of MRT-2359 in NE or AR negative /c-MYC low prostate cancer models

In vitro profiling indicated limited MRT-2359 sensitivity in AR negative / c-MYC low prostate cancer cell lines, whereas the sensitivity of a NE prostate cancer cell line was unclear. Therefore, MRT-2359 in vivo activity was assessed in subcutaneous xenograft models of the NE cell line NCI-H660(1) or the AR negative / c-MYC low cell line PC3 (2).

1		NCI-H660		2	PC3
SEM)	2000	neuroendocrine	Vehicle		AR negative / c-MYC low
no further treatment		2000
	SEM)
	MRT-2359, 10 mg/kg QD
		Vehicle
		MRT-2359, 3 mg/kg 5 days on / 9 days off

2	KEGG_RIBOSOME	82	2.35

GSEA was run on a pre-ranked list of genes differentially expressed at baseline between sensitive and resistant cell lines using n=205 MSigDB Hallmark and KEGG pathway gene sets with Broad GSEA software. The top 2 positively enriched sets are shown above. An enrichment plot for the KEGG ribosome gene set is shown on the right. Similar enrichment was observed for "mTOR dependent" genes (Hsieh et al, Nature 2012 and Park et al, Cell Reports 2017; gene sets are not part of MSigDB).

Abbreviations: GSEA, gene set enrichment analysis; NES, normalized enrichment score.

ribosomal genes.

• This observation suggests an altered

state of global translation (see

"Introduction"), potentially linked to

high c-MYC, leading to increased

sensitivity to translation blockade via

GSPT1 degradation.

01	.	10	100	1000	10000
.	0
0

MRT-2359; nM

no transcriptomics data available

0

-10-5 0 5 10

fold change (log2)

±	1500
mean																																															MRT-2359, 10 mg/kg 5 days on / 9 days off	±	1500					MRT-2359, 10 mg/kg QD
																																															MRT-2359, 30 mg/kg 5 days on / 9 days off	mean					MRT-2359, 10 mg/kg
,																																																						5 days on / 9 days off
3
																																																	,
																																																	3

Summary and Future Development

CellTiter-Glo assays, 72h incubation with MRT-2359	Differential gene expression analysis at baseline between
	sensitive (n = 4) and resistant (n = 3) cell lines shown in panel 2

volume(mm	1000	(mm	1000

• In vitro cancer cell line profiling of the GSPT1 MGD MRT-2359 revealed marked sensitivity in a subset of

prostate cancer cell lines associated with readily measurable biomarker candidates (AR + c-MYC or NE).

3

absolute IC50, log10(µM)

2
		DU145
1	NCIH660	PC3
0
		MDAPCa2b
-1		LNCaP (clone FGC)
		22RV1
		VCaP

-2

AR-positive

AR-negative neuroendocrine**

• LASCPC01 not shown (MYC mRNA N/A)

•	Four AR-positive /MYC-high prostate cancer
	cell lines are very sensitive to MRT-2359
	(absolute IC50 ~50-150 nM)
•	Two AR-negative /MYC-low prostate cancer
	cell lines are resistant to MRT-2359 (absolute
	IC50 > 10 µM)
•	Mixed in vitro response in the two tested

	500					TV = 0 in 9/9 mice treated with	Tumorvolume	500
Tumor
					MRT-2359 at 10 mg/kg QD
0						0
	0	10	20	30	40	50		0	10	20	30

Time post treatment initiation (days)

Time post treatment initiation (days)

NCI-H660,regression (%) on day 28: MRT-2359, 10 mg/kg QD, -100; 3 mg/kg 5 days on / 9 days off: -7; 10 mg/kg 5 days on / 9 days off, -78.Abbreviations: as above; NE, neuroendocrine.

Despite apparent moderate maximal effect in a 3-day in vitro assay, MRT-2359 led to profound regression of NCI-H660 xenografts in vivo. This observation is reminiscent of the activity of MRT-2359 in neuroendocrine lung cancer models1. In line

• Treatment of MRT-2359 sensitive prostate cancer cell lines in vitro led to a loss of c-MYC and, to a lesser

extent, AR (including AR-V7) proteins.

• Marked tumor regression upon treatment with MRT-2359 as single agent was observed in three cell line

derived xenograft models that were positive for the biomarker candidates discovered in vitro, but no efficacy

was seen in a biomarker negative xenograft model.

• In an AR positive model with minimal expression of the variant AR-V7, combinations of MRT-2359 and

enzalutamide were more efficacious than the respective single agent treatments.

4

5

6

7

8

MYC mRNA, log2(TPM+1)

Absolute IC50 (data above) vs MYC mRNA expression (DepMap)

neuroendocrine (NE) prostate cancer cell lines,

but low relative IC50 in both cases

with in vitro data, no significant MRT-2359anti-tumor activity was detected in the AR negative / c-MYC low xenograft model PC3.

• These data warrant clinical investigation of MRT-2359 as single agent or in combination with an AR

antagonist in patients with prostate cancer.

References: 1. Gavory et al. AACR Annual Meeting 2023; 2. Gavory et al. AACR-NCI-EORTC 2021

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All authors are employees of Monte Rosa Therapeutics