Disclaimer This presentation has been prepared by Mereo BioPharma Group plc (the "Company") solely for your information and for the purpose of providing background information on the Company, its business and the industr

Mereo BioPharma Group plc NASDAQ: MREO

March 14 2022 Alvelestat (MPH966) R&D Day Exhibit 99.1

Disclaimer This presentation has been

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Summary The ASTRAEUS 12-week trial is

on track to deliver top-line Phase 2 data for alvelestat in AATD patients with lung emphysema in early Q2 2022* Primary endpoints evolved to three key biomarkers on the pathogenic pathway of the lung disease including detection of neutrophil

elastase levels in patients More comprehensive - allows investigation of a number of causal pathway biomarkers for further development In-vivo neutrophil elastase assay more akin to "functional PK" in-vitro assays used as a primary

end-point in Phase 2 studies for AAT augmentation Follows on from Mereo's development of the biomarker strategy and the Type C meeting held with the FDA The ATALANTa Phase 2 study will currently read-out in early 2023 and is

complementary to ASTRAEUS Alvelestat has demonstrated a clean safety profile in all studies to-date with a tendency for headaches managed through dose escalation* Plan to engage in discussions with the FDA on Registrational trial design for

AATD in end of Phase 2 meeting in 2H 2022 Alvelestat has potential in other indications Demonstrated impact on biomarkers of elastin breakdown and fibrosis in BOS following allogeneic hematopoietic stem cell transplantation, with encouraging

pulmonary function outcomes. Phase 2 to be initiated in earlier stage patients, with clinical endpoints. Demonstrated a clinically meaningful effect in short term 5-10 day dosing in hospitalised COVID-19 supports anti-inflammatory effects

on top of those achieved by high dose corticosteroids *Note: Uncleaned data. Final values may differ from those presented here.

Upcoming Key Milestones &

Opportunities Upcoming Milestone For Core Programs Product Candidate Indication 2022 2023 2024 Partner Next Milestone Etigilimab Solid tumors Phase 1b/2 full enrolment and data Phase 2 cohort expansion Alvelestat AATD AATD

Phase 2 top-line data BOS Phase 2 initiation BOS Setrusumab Osteogenesis imperfecta Initiation of pivotal study pediatric & young adults (5-25yrs old) Initiation of Phase 2 children <5 yrs old) Phase 2 ASTRAEUS*

Phase 1b Phase 1b/2 basket study with potential cohort expansion Pediatric Phase 2b/3 fracture study Pediatric Phase 2 children <5 years Non-core Programs Navicixizumab has been partnered with OncXerna for further development. Received a $2M CMC

milestone Leflutrozole and acumapimod are currently under partnering discussions. Next Milestone: Partnership agreement Phase 2 Phase 2 ATALANTa Projected milestone *ASTRAEUS is a proof-of-concept phase 2 study

ASTRAEUS Phase 2 Study in

AATD-Associated Emphysema Randomisation Alvelestat Placebo bid ~ 36 Alvelestat Dose 1 120 mg bid ~ 22 Dose escalation design 12-week dosing period, 4 weeks FU Alvelestat Dose 2 ~ 41 Trial Population Age 18 and 80 years Pi*ZZ,

PiZ Null, PiNull genotype/phenotype, other rare types FEV1 20% predicted A randomized double blind-placebo-controlled study in patients na ve to augmentation or following a 6-month wash-out period. Total of 99 patients

enrolled. Chief Investigator - Prof. Rob Stockley IDMC -both doses deemed safe to proceed Preferential recruitment to highest dose per protocol

ASTRAEUS Phase 2 Initial Endpoints

Primary Endpoints Within individual % change from baseline in plasma desmosine/isodesmosine at end of treatment compared to placebo to week 12 Secondary and Exploratory Endpoints Blood Neutrophil Elastase activity Blood A -Val360

levels Safety and tolerability Lung damage and inflammation biomarkers Pharmacokinetics St. George's Respiratory Questionnaire Spirometry including - Forced expiratory volume in 1 second (FEV1), FVC and FEF25-75

ASTRAEUS Demographics And Baseline AATD

Characteristics ASTRAEUS RAPID* All, (N=99) A1PI(n=93) Placebo (n=87) Age years Mean SD (Range) 57.3 10.1 (26-75) 53.8 6.9 52.4 7.8 Sex Male (%) Female (%) 39(40%) 60(60%) 48(52%) 45(48%) 50(57%) 37(43%) Race

White (%) 95(96%) 93(100%) 87(100%) FEV1 % predicted Mean SD 56.7 20.7 47.4 12.1 47.2 11.1 A1AP concentration m Mean SD (Range) 3.9 1.6 (0.04-9.0) 6.38 4.62 5.94 2.42 Mutation

status All ZZ ^NR ^NR Time from AATD diagnosis years -Median (Range) 9 (1-42) ^NR ^NR $ Preliminary uncleaned data *Chapman et al. (2015) Lancet. 2015 Jul 25;386(9991):360-8 ^NR- Not Reported Similar to other baseline data

for randomized control trials in severe AATD patients$ Note: Uncleaned data. Final values may differ from those presented here.

Plasma Desmosine as a Biomarker of

Proposed Pathogenic Mechanism (Elastosis) Marker of Effect on Target Tissue Measure of elastolytic rate (desmosine/isodesmonsine = "desmosine") Consistently increased in AATD-associated lung disease (emphysema) Addresses question of whether NE

inhibition alone modifies a marker within the pathogenic pathway Desmosine increased in diseases associated with increased NE and shows response to NE suppression with alvelestat in signal-seeking clinical studies Cystic Fibrosis: ~19%

reduction alvelestat (n=26) compared to placebo (n=29) (p=0.105) by 4 weeks$ Bronchiectasis: ~10% reduction in alvelestat compared to placebo (p= 0.120) by 4 weeks# Bronchiolitis Obliterans Syndrome (BOS): decrease of ~ 16% from baseline by week 8

(p=0.066)^ $Elborn et al., (Log transformed data on file) 2012; #Stockley et al., 2013; ^ Im et al., 2021

Baseline Desmosine Levels In ASTRAEUS Consistent With Other Studies Data, including therapeutic response, available in AATD Correlate with clinical measures of disease severity (FEV1), respiratory function (diffusion factor) and structure (lung

density) in AATD population Elevated in AATD across different studies Responsive to AAT replacement (~ 6.5% reduction at 3 months) in RAPID study$ Elevated (blinded) baseline desmosine levels in ASTRAEUS, consistent with other studies of

severe AATD ASTRAEUS baseline = 0.386 ng/ml (SD 0.137) RAPID study baseline = 0.365 ng/ml (SD 0.101) Healthy volunteers = 0.21ng/ml (SD 0.03) $ Ma et al Chronic Obstr Pulm Dis. 2017; 4(1): 34-44 ** Albarbarawi et al. (2013)

Bioanalysis. 2013 Aug;5(16):1991-2001 Note: Uncleaned data. Final values may differ from those presented here.

Monitoring Neutrophil Elastase (NE)

Levels In Patients With AATD "Functional PK" assays used in AAT augmentation measure NE inhibiting capacity of serum to quantitate level of "biologically active" AAT Does not measure NE activity within a

patient and cannot be used as a measure of ongoing in vivo suppression Measurement of elastase in AATD to monitor therapeutic intervention is challenging: Limited sensitivity of assays for blood NE activity NE activity assays

available for sputum, but minority with AATD produce sputum Increased sensitivity of an established NE activity method (ProAxsis ) now enables measurement of blood elastase activity in patients enrolled in ASTRAEUS In

Measurement Of Target Engagement -

Activity-based NE Immunoassay For In-vivo Blood Monitoring Of NE Suppression The ProteaseTag activity-based immunoassay specifically measures active neutrophil elastase levels Improved sensitivity, enables detection in

blood supports potential detection of therapeutic effect. Significant difference between the active NE concentrations in each group (p = <0.0001) Active NE (ng/ml) ASTRAEUS Baseline Blinded data (N=82) Healthy Subjects Recent data

(N=39) Mean 63.6 9.6 Median 40.7 0 Range 0-685.6 0-104.2 Note: Uncleaned data. Final values may differ from those presented here.

Simultaneous incubation

Alvelestat/elastase *AZD9668:

Plasma A -Val360 Blinded

Baseline Levels Raised In ASTRAEUS AATD Population Mereo Biopharma Group plc Blinded baseline level ASTRAEUS mean 15.05nM (SD 4.81) Similar to levels in AATD Registry study in PiZZ (Carter et al 2013) Positive correlation between desmosine

and A -Val360 levels at baseline in ASTRAEUS Note: Uncleaned data. Final values may differ from those presented here.

No Safety Signals Identified To-date

In AATD Patients (Data Blinded To Mereo) Independent Data Monitoring Committee reviews unblinded safety data ~ quarterly. There have been no safety signals detected on adverse event or lab monitoring, including in infectious events, liver,

hematology or ECG review. Most commonly reported treatment-emergent adverse event (TEAE) has been headache- considered a tolerability issue and not a safety issue for alvelestat. Headache is a known side effect of alvelestat and was

most frequent in the highest dose arm. Dosing amended to include a within-participant dose escalation step which reduced the frequency/severity of headache Note: Uncleaned data. Final values may differ from those presented

Linking Biomarkers To Pathological

Pathway AATD-Lung Disease Pathogenic Pathway Neutrophil Elastase Activity Unopposed NE Alveolar Elastin Breakdown Lung Tissue Loss/Emphysema Increased neutrophil elastase activity Increase in neutrophil elastase-driven target

(fibrinogen) breakdown Elastin breakdown fragments A -Val360 Blood NE Desmosine CT Density/FEV1 Alvelestat

ASTRAEUS Phase 2 Revised Endpoints

Primary Endpoints Within individual % change from baseline up to end of treatment within treatment arm and in comparison, to placebo up to week 12 in: Plasma desmosine/isodesmosine levels Blood Neutrophil Elastase activity Blood

A -Val360 levels Secondary and Exploratory Endpoints Safety and tolerability Lung damage and inflammation biomarkers Pharmacokinetics St. George's Respiratory Questionnaire Spirometry including - Forced expiratory volume in 1

second (FEV1), FVC and FEF25-75 Exacerbations

Alvelestat (MPH966) For The

Treatment of ALpha-1 ANTitrypsin Deficiency "ATALANTa" (Investigator-initiated - Principal Investigator Prof Mark Dransfield, UAB) Randomisation Alvelestat Placebo bid N=33 Alvelestat 120 mg bid N=33 N = 66 1:1 active to

placebo 12 week dosing period Trial Population Age 18 and 80 years PiZZ, PiSZ, PiZ Null, or PiNull genotype/phenotype Emphysema, FEV1 25% predicted Not currently receiving augmentation OR on stable augmentation for at least

12 weeks prior to screening Primary Endpoints Within-individual % change in plasma desmosine/isodesmosine (week 12) Safety and tolerability Secondary Endpoints Blood A -Val360, Neutrophil elastase Protease neo-epitopes Collagen

peptides/chemoattractants Pro-inflammatory cytokines Exploratory Endpoints PK/PD Spirometry PROMs

Consistent Supportive Safety Profile Of Alvelestat In Patients With AATD 10 active sites in USA 37 patients randomized No SAEs reported in the study to date No clinically important abnormalities in hematology and clinical chemistry

reported to date 6 monthly DSMB meeting (last Dec 2 2021) "We reviewed AEs and safety data and noted no concerning trends" Note: Uncleaned data. Final values may differ from those presented here.