AXO-AAV-GM1 6-Month Topline Safety and Efficacy Low-Dose Cohort (1.5x1013 vg/kg) AXO-AAV-GM1: GENE THERAPY FOR GM1

Only Clinical-Stage Gene Therapy Program Targeting the Full Spectrum of Patients with Type I and Type II GM1 Gangliosidosis

GM1 Gangliosidosis AXO-AAV-GM1 Gene Therapy Rationale

• Fatal, pediatric lysosomal storage disorder with monogenic • One time delivery of a functional copy of the GLB1 gene via autosomal recessive inheritance pattern that affects ~450- intravenous administration of AAV9 vector system 900 US+EU patients • Intravenous delivery addresses the multisystem • Single gene (GLB1) defect leads to decreased β-galactosidase manifestations of GM1 gangliosidosis enzyme activity and accumulation • Only gene therapy to demonstrate restoration of wild-type • Disease hallmark is progressive due to survival in a naturally-occurring GM1 feline model storage of GM1 ganglioside in lysosomes • Positive six-month enzyme activity and preliminary clinical • Late infantile/juvenile (Type II) have a life expectancy of 10- outcomes data in late-infantile/juvenile (Type II) children 20 years treated with low-dose gene therapy (1.5x1013 vg/kg) • Currently no treatments available • High dose cohort (4.5x1013 vg/kg) initiated in November 2020; two patients now dosed without complications

2 GM1 Gangliosidosis Background GM1 GANGLIOSIDOSIS: Part of a spectrum of neurodegenerative lysosomal storage disorders with monogenic autosomal recessive inheritance pattern

Lysosomal Storage Diseases GM1 Gangliosidosis

• >50 related disorders • Mutation in the GLB1 gene leads to impaired β- galactosidase enzyme • Frequency ~1:7,700 total • Without β-galactosidase GM1 builds up in the • Two-thirds are neurodegenerative, including lysosome of the cell GM1 gangliosidosis, Tay-Sachs, and Sandhoff disease • Accumulation of GM1 ganglioside in multiple organ systems causes significant defects: • Naturally occurring large animal models, including felines and sheep • Severe progressive neurological decline • Hepatosplenomegaly • Blindness and ocular deficits • Osteoporosis and skeletal dysfunction • Naturally occurring feline model

4 THE LYSOSOME IS AN IMPORTANT REGULATOR OF CELL FUNCTION

Ballabio A, Bonifacino JS. Lysosomes as dynamic regulators of cell and organismal homeostasis. Nat Rev Mol 5 Cell Biol 2020; 21(2): 101-118 Courtesy Dr. Cynthia Tifft STORAGE OF GM1 IS DUE TO A DEFICIENCY IN β-GALACTOSIDASE

• GM1 ganglioside is an integral part of the lysosomal membrane • The first step in degradation requires the enzyme β-galactosidase • β-galactosidase removes the first sugar (galactose) from the larger molecule • Without β-galactosidase, GM1 builds up in the lysosome of the cell

6 Courtesy Dr. Cynthia Tifft 4 GM1 GANGLIOSIDOSIS SEVERITY VARIES ACROSS DISEASE TYPES

Type I (Infantile) • Onset <6 mo • Skeletal dysplasia • Hepatosplenomegaly • Developmental arrest • Hypotonia Type II (Late-Infantile) • Cherry red macula • Onset 1-2 yrs • Seizures • Difficulty walking • Death at <2 years Type II (Juvenile)

ty • Variable skeletal i • Onset 2-5 yrs disease • Impaired ambulation • Decreased cognition

sease • Dysarthria i ever • Seizures • Variable skeletal D S • Death in mid-teens disease • Decreased cognition Type III (Adult) • Survival into 3rd decade • Adult onset • Gait disturbance • Dystonia, dysarthria • Decreased cognition • Age at death variable

Enzyme Activity

7 Regier DS, Tifft CJ. 1993. GLB1-Related Disorders. In: Adam MP, Ardinger HH, Pagon RA, Wallace SE, Bean LJH, Stephens K, Amemiya A, editors. GeneReviews. Seattle (WA): University of Washington, Seattle. CLINICAL, SKELETAL, NEUROIMAGING, AND BIOCHEMICAL FINDINGS IN GLB1-RELATED DISORDERS

GM1 Gangliosidosis Finding Type I Type II Type III Infantile Late Infantile Juvenile Chronic/Adult Onset of symptoms <1 year 1-2 years 3-10 years 10+ years Eye findings CRS CC CC +/– CC Motor abnormalities + + + Extrapyramidal Hepatosplenomegaly + +/– +/– – Cardiac involvement +/– +/– +/– +/– Coarse facial features +/– – – – Skeletal findings + +/– +/– – Seizures + + +/– – Neuroimaging PA PA PA +/– mild atrophy

Symbols and abbreviations: (+): present finding; (-): absent finding; (+/–): variable finding in subject population; CRS: cherry red spot; CC: corneal clouding; PA: progressive atrophy

8 WELL-UNDERSTOOD BIOLOGY FOR GM1 GANGLIOSIDOSIS MAKES IT AN ATTRACTIVE TARGET FOR GENE THERAPY

GLB1 Gene

β-galactosidase GM1 Ganglioside GM2 Ganglioside GM1 Gangliosidosis

Underlying neurobiology Clinically relevant Broad distribution achieved supports the use of gene biomarkers paired with via one-time administration therapy for long-term well-accepted and cross-correction enzyme restoration neurodevelopmental mechanism measures

9 CROSS-CORRECTION PRINCIPLE ALLOWS SPECIFIC EXTRACELLULAR LSD ENZYMES TO TRANSDUCE THE LYSOSOMES OF OTHERWISE ENZYME-DEFICIENT CELLS

10 Preclinical Summary INTRAVENOUS DELIVERY TO ADDRESS THE MULTISYSTEM MANIFESTATIONS OF GM1 GANGLIOSIDOSIS

Cisterna magna Intravenous Delivery Delivery

Invasiveness Medium Minor

CNS Outer surface of Whole brain and spinal Distribution and spinal cord* cord

CEREBRAL CARDIAC ABNORMALITIES ATROPHY • Heart Low, highly variable • Liver-Spleen HEPATOSPLENOMEGALY SKELETAL DYSPLASIA Body vector delivery to body • Gastrointestinal Distribution organs • Musculoskeletal CHERRY-RED SPOT • Peripheral nerves

12 *Data on file ONLY GENE THERAPY TO DEMONSTRATE RESTORATION OF WILD-TYPE SURVIVAL IN A GM1 FELINE MODEL1

GM1 Felines Treated with 1.5E13 vg/kg via Intravenous Injection at 5 Weeks of Age

10

9

8

7

6 IV infusion 5

4

3 Clinical Rating Score Rating Clinical

2

1

0 1 6 11 16 21 26 31 36 Age (months)

13 Normal GM1 feline + one-time gene therapy GM1 feline 1: UMass Data on File; WPRE included in construct administered to felines Note: Felines were treated with the first-generation construct and the AXO-AAV-GM1 vector has been optimized prior to dosing patients in the ongoing clinical trial. WIDESPREAD Β-GAL ENZYME ACTIVITY ACHIEVED ACROSS THE CNS IN A GM1 FELINE MODEL WITH IV ADMINISTRATION

Broad distribution of β-gal staining across the brain and spinal cord with IV dosing of 1.5E13 vg/kg

Significant long-term coverage with strong expression

14

McCurdy, et al, Sci Transl Med. 2014;6(231):231ra48. INDEPENDENT STUDY CONFIRMS AAV9 IV ROUTE OF ADMINISTRATION HAS SUPERIOR CNS DISTRIBUTION IN GM1 FELINES

All feline animals were treated with 1.5e13 vg/kg and used a similar vector backbone

1: Data from Auburn University presented at ASGCT 2020 Meeting Note: Felines were treated with a vector that includes a WPRE component; AXO-AAV-GM1 does not include this component. INDEPENDENT STUDY DEMONSTRATES AAV9 GENE THERAPY IV ROUTE OF ADMINISTRATION IS MOST EFFECTIVE IN EXTENDING SURVIVAL AND IMPROVING CLINICAL FUNCTION

All feline animals were treated with 1.5e13 vg/kg and used a similar vector backbone

16 1: Data from Auburn University presented at ASGCT 2020 Meeting Note: Felines were treated with a vector that includes a WPRE component; AXO-AAV-GM1 does not include this component. AAV9 IV ROUTE OF ADMINISTRATION DEMONSTRATED TO BE MOST EFFECTIVE IN THE FELINE MODEL

CONCLUSIONS

17 1: Data from Auburn University presented at ASGCT 2020 Meeting Note: Felines were treated with a vector that includes a WPRE component; AXO-AAV-GM1 does not include this component. EXTENSIVE PRECLINICAL DATA SUPPORT DIFFERENTIATED CLINICAL APPROACH

AAV9 outperforms AAVRh10 across Optimized Vector administration routes

Independent preclinical studies Intravenous administration has superior CNS show consistent superiority of Optimized Delivery distribution and βGal activity with potential to vector, delivery and efficacy with address multisystem manifestations AXO-AAV-GM1 IV delivery

Restoration of white and grey matter, and cortical Stabilization of neuromuscular Restored Cortical volume, as measured by MRI observed in feline decline and survival prolongation Volume model observed in naturally occurring feline model

Only gene therapy to demonstrate restoration of wild Superior Outcomes type survival in naturally occurring feline model

18 Study Design, Assessments, and Baseline Demographics AXO-AAV-GM1: CLINICAL STUDY DESIGN

STAGE 1: DOSE-RANGING Ongoing STAGE 2: EFFICACY STUDY

6-month data readout High-dose Cohort December 2020 4.5 x 1013 vg/kg

Low-Dose Cohort 1.5 x 1013 vg/kg Type I (infantile) and Type II (juvenile)

Stage 1 and Stage 2 will include a mix of Type I (infantile) and Type II (late-infantile/juvenile)

ROUTE OF • Intravenous ADMINISTRATION • Safety and tolerability • Developmental changes and disease progression measured by: Vineland-3 Adaptive Behavior Scales, Bayley Scales of Infant Development (Type I only) and Clinical Global Impressions Scale CLINICAL • Changes to MRI/MRS MEASURES (TYPE II) • Motor function and disease severity • Skeletal survey, swallow/speech assessment • Exploratory objectives include systemic and CSF biomarkers and neurological symptoms

• High dose cohort (4.5x1013 vg/kg) initiated in November 2020; two patients now dosed without complications NEXT STEPS • Continued dosing of infantile (Type I) and late-infantile/juvenile (Type II) patients in high-dose cohort of ongoing study • Obtain data in both Type I and Type II children as part of clinical and regulatory strategy to obtain broad indication 20

ClinicalTrials.gov Identifier: NCT03952637 AXO-AAV-GM1: CLINICALLY MEANINGFUL ASSESSMENTS OF THERAPEUTIC RESPONSE

Growth Scale Value (GSV) Vineland-3 Adaptive Behavior Assessment (VABS-III) • GM1 presents with regression in function or loss of achieved milestones • Vineland-3 is a measure of adaptive behavior that is widely used to assess intellectual ability in Score patients with neurodevelopmental disabilities • Evaluates communication, daily living, and social skills Ability • VABS-III demonstrates predictable decline in natural history studies and stabilization has not GSV Score has predictable relationship to ability been reported previously.

Clinical Global Impression Scale (CGI) Biomarkers of Disease Activity • Rates severity and improvement based on clinical opinion from 1-7 • Level of residual enzyme correlates inversely with the severity of • 1 = “normal” or “very much improved since initiation of treatment” the disease • “7” = “among the most extremely ill patients” or “very much worse since the initiation of treatment” • 10-20% enzyme activity threshold thought to aid in clearance of • Stabilization in CGI relative to natural history has not been reported stored lysosomal substrates and associated with slower disease previously progression of various lysosomal storage disorders. • Lower the level of enzymatic activity, the more severe the 1 2 3 4 5 6 7 Normal, not Minimally Mildly ill Moderately Markedly Severely Very phenotype at all ill ill ill ill ill severely ill

21 (1) Lang et al., Mol Genet Metab, 2020; (2) NIH data on file; (3) Utz et al., Mol Genet Metab, 2017; (4) Yogalingam et al., J. Biol. Chem. 2019. DEMOGRAPHICS AND BASELINE CHARACTERISTICS

Subject Sex/Race Disease Age (in months) Biallelic Documented Phenotype Impairment Of Change In History Use Of Condition Mutations Deficiency Of β- Consistent Motor Skills Walking Of Falls Assistive in GLB1 Gal Enzyme With GM1 Pattern Device Diagnosis Onset Dose Gross Fine Administered 003 M/White IIb-Juvenile 12 29 45 Yes Yes Yes No Yes Yes No No Onset 004 M/Middle IIa-Late Infantile 23 28 32 Yes Yes Yes Yes Yes Yes Yes Yes Eastern Onset 006 M/White IIa-Late Infantile 44 16 68 Yes Yes Yes Yes Yes Yes Yes No Onset 007 F/White IIa-Late Infantile 29 35 47 Yes Yes Yes Yes Yes Yes Unknown No Onset 008 F/White IIa-Late Infantile 29 35 47 Yes Yes Yes Yes Yes Yes Yes No Onset

22 OVERALL SAFETY SUMMARY (6-MONTH DATA)

• AXO-AAV-GM1 was generally safe and well-tolerated at the low dose (1.5x1013 vg/kg) delivered intravenously in Type II (late-infantile and Juvenile) patients.

• There have been no serious adverse events (SAEs) related to gene therapy. o One SAE was described: a single patient experienced bacterial sepsis due to a PICC line infection, which was considered to be unrelated to the investigational drug product, and which resolved within a few days following line removal and administration of IV antibiotics. • The most common adverse events were considered mild to moderate. Transient AST elevations were observed in 4 subjects, none of which required clinical intervention or had associated clinical sequelae. There were no other adverse events indicative of impaired liver function including serum bilirubin, GGT, and ALT. No clinically relevant changes were observed in platelet count. • The favorable safety profile in the low-dose cohort supports continued enrollment of patients in the high- dose cohort (4.5x1013 vg/kg), in which two patients have now been dosed without complications.

23 AXO-AAV-GM1: 6-Month Data Readout Biomarkers β-GALACTOSIDASE ENZYMATIC ACTIVITY ASSAY DEVELOPMENT

Normal Synthetic fluorescent substrate • Assay range – 27.9 to 0.37 nmol/hr/mL serum • Linear through the assay range with R2 values >0.99 in each of 6 validation runs

Beta-gal • QC variability in each of 6 validation runs had <10% CV at each High, Medium, and Low QC samples Deficient or enzyme variant • Normal reference range for Beta-gal Synthetic fluorescent substrate activity established with 30 presumed normal adult serum samples • Clinical reference range for Beta-gal activity established with 6 natural history samples Beta-gal variant

25 ^ These 30 samples provide the reference range for enzyme activity in this cohort β-galactosidase ENZYME ACTIVITY IN SERUM ACROSS 9 DISTINCT TIMEPOINTS Increase from baseline between 71-138% (mean: 110%) during the 6-month observation period

160 N = 5 138 140 132 123 123 120 114 109 106 Cohort Mean: 110% 100

80 74 71

60 GALACTOSIDASE ENZYME ENZYME ACTIVITY GALACTOSIDASE - 40 % INCREASE FROM BASELINE IN SERUM BETA

20

0 Day 7 Day 14 Day 21 Day 30 Day 45 Day 60 Day 75 Day 90 Day 180 Data Collection Timepoint 26 % Change from Baseline β-galactosidase ACTIVITY IN SERUM BY SUBJECT: Enzyme activity restored to 23-57% (mean: 38%) of normal* reference levels at Month 6

60.0 56.5 N = 5

50.0 48.5

40.0 Cohort Mean: 38% 37.3

30.0 25.7 22.7

20.0 % of Normal Reference Levels Reference Normal % of

10.0

0.0 DAY 180 DAY 180 DAY 180 DAY 180 DAY 180 001-003 001-006 001-008 001-004 001-007 27 *The reference level was defined by the lowest level of enzyme activity in serum from 30 healthy adult volunteers using the same validated assay of beta-galactosidase enzyme activity as was used to assess the patients in the study. BIOMARKER SUMMARY: Positive Increase in β-Galactosidase Serum Levels Over Six Months

• Serum beta-galactosidase enzyme activity was sampled at 9 distinct timepoints between Day 7 and Month 6 • The proportional increase from baseline across the patients ranged between 71-138% (mean: 110%) during the 6-month observation period, representing an approximate doubling in enzyme activity after gene transfer. At Month 6, serum enzyme activity increased by an average of 71% from baseline (range: 33%-127%) across the five patients. • At month 6, serum enzyme activity on average was restored to 38% of normal reference levels at Month 6, with individual patients ranging from 23-57% of these normal levels. • The reference level was defined by the lowest level of serum enzyme activity consistent with a normal clinical presentation in 30 healthy adult volunteers using a validated assay of beta- galactosidase enzyme activity. • Cerebrospinal fluid (CSF) samples were collected in all patients over the 6-month period. Development and validation of biomarker assays for CSF is currently ongoing.

28 AXO-AAV-GM1: 6 Month Data Readout Clinical and Functional Outcomes

29 GM1 NATURAL HISTORY: MOTOR FUNCTION EXHIBITS PREDICTABLE, AGE-RELATED DECLINE IN UPRIGHT AND FLOOR MOBILITY SCORE

Score Description 5 Ambulation is normal for age 5 4 Independent ambulation, may be unsteady 4 Ambulates independently with mobility aid 3 Scale due to unsteady gait 3

obility R² = 0.4267 Ambulates only with the assistance of Mobility 2 M 2 another person (with or without a 1

mobility aid) due to unsteady gait Upright

Able to stand but not walk, purposeful 0 1 0 5 10 15 20 25 30 movement by scooting or crawling Age Upright Non-ambulatory, wheelchair or stroller 0 dependent

Score Description 5

5 Crawls in 4-point independently 4 Scale 4 Scoots independently when sitting 3 R² = 0.1865

obility 2 3 Sits without support M

Floor Mobility 1 2 Sits with support 0 0 2 4 6 8 10 12 Floor 1 Rolls independently 30 Age

0 Unable to roll FLOOR AND UPRIGHT MOBILTY SCORES BY SUBJECT AT MONTH 6 All subjects demonstrate disease stability

5 5 5 5 5 5 5 5 5 55 5 5 5 5 5 5 5 5 5 4 4 4 4 4 4

3 3 3 3 3 3

2 2 2 2 2 2 Floor Mobility Score 1 Upright Mobility Score 1

0 0 24 34 44 54 64 74 84 24 34 44 54 64 74 84 Age (in months) Age (in months) 001-003 001-004 001-006 001-007 001-008 001-003 001-004 001-006 001-007 001-008

Motor Scoring- Floor Mobility: Visit Schedule: Upright Mobility Scale: 0 = Unable to roll Screening 0 = Non-ambulatory, dependent for wheeled mobility (WC or stroller) 1 = Rolls Independently Day 30 1 = Able to pull to stand, unable to ambulate 2 = Sits with support Day 90 2 =Ambulates only with the assistance of another person (with or without a 3 = Sits without support Day 180 mobility aid) due to unsteady gait 31 4 = Scoots independently when sitting 3 =Ambulates independently with mobility aid due to unsteady gait 5 = Crawls in 4-point independently 4 =Independent ambulation, may be unsteady 5 =Ambulation is normal for age CLINICAL GLOBAL IMPRESSION SCALE (CGI) AT MONTH 6 Improved in 4/5 patients and remained stable in the 5TH child over evaluation period.

7 7

6 6

5 5 4 4 4 4 4 4 4 4 4 4 at Baselineat 3 3 3 3 3 3 3 3 3 3 3 3 3 2

Severity 2 2 Compared to Baseline - - CGI 1 CGI 1

0 0 001-003 001-004 001-006 001-007 001-008 001-003 001-004 001-006 001-007 001-008

D30 D180 D30 D180 D30 D180 D30 D180 D30 D180 SCREENING SCR SCR SCR SCR D90 D90 D90 D90 D90 (SCR)

CGI Baseline Severity : 001-003 001-006 CGI Compared to Baseline: 1 = Normal 001-004 001-007 1 = Very much improved 2 = Borderline ill 001-008 2 = Much improved 3 = Mildly ill 3 = Minimally improved 4 = Moderately ill 4 = No change 5 = Markedly ill 5 = Minimally worse 6 = Severely ill 6 = Much worse 32 7 = Among the most extremely ill patients. 7 = Very much worse VINELAND-3: GROWTH VALUE SCORES DEFINITIONS BY DOMAIN

33 VINELAND-3: SUBDOMAIN GROWTH SCALE VALUE SCORES MONTH 6 (1/2)

Receptive Communication Expressive Communication Written Communication[1]

180 180 166 180 154 142 150 150 150 136 121 137 117 117 133 135 125 120 120 134 120 114 133 107 109 125 126 118 113 90 109 109 106 90 90 100 87 87 87 82 82 60 60 60 42 69 40 47 29 42 30 30 30

Communication 40 42 25 32 10 29 29 0 0 0 24 36 48 60 72 84 24 36 48 60 72 84 24 36 48 60 72 84 Personal Domestic[1] Community[1]

150 180 180

117 150 150 120 110 110

100 100 120 120 96 106 90 102 101 100 Growth Scale Value Score Value Scale Growth 99 93 90 90

76 75 60 70 45 60 41 60 44 36 36 37 34 37 37 34 42 30 39 39 34 30 42 42 30 39 38 Daily Living Skills Living Daily 38 36 34 31 10 10 0 0 0 24 36 48 60 72 84 24 36 48 60 72 84 24 36 48 60 72 84

Visit Schedule: 34 Age in Months Screening, Baseline, Day 180 001-003 001-006 001-008 [1] Domestic, Community and Written subdomains are applicable to subjects 3 years and older. Subject 004 turned 3 years old between Day 90 and Day 180. 001-004 001-007 VINELAND-3: SUBDOMAIN GROWTH SCALE VALUE SCORES MONTH 6 (2/2)

Interpersonal Relationships Play and Leisure Time Coping Skills

180 180 180

150 150 150

113 120 106 120 106 108 120 97 96 99 93 97 95 90 91 90 93 92 90 87 68 97 96 99 96 67 92 93 93 95 88 57 83 60 60 60 59 73 73 70 57 55 57 57 56 62 62 64 58 59 Socialization 51 30 30 30 46 45 46

0 0 0 24 36 48 60 72 84 24 36 48 60 72 84 24 36 48 60 72 84 Gross Fine

180 180 155 148 148 001-003 001-006 150 150

120 118 117 001-004 001-007 128 126 120 124 120 128 125 126 112 112 001-008 119 105 Growth Scale Value Score Value Scale Growth 118 118 117 112 112 90 90 108 108 100 86 81 60 60 75 72 Motor 68 54 30 30

0 0 24 36 48 60 72 84 24 36 48 60 72 84

35 Visit Schedule: Age (in Months) Screening, Baseline, Day 180 AXO-AAV-GM1 LOW-DOSE TYPE II PATIENT COHORT: 6-MONTH SUMMARY • Generally well-tolerated with a favorable safety profile • Serum beta-galactosidase enzyme activity increased in all patients at all timepoints between Day 7 and Month 6, representing an approximate doubling in enzyme activity after gene transfer • All five children demonstrated signs of clinical disease stability across multiple measures of neurodevelopment • High-dose cohort (4.5x1013 vg/kg) for Type II patients initiated in November 2020 with two patients dosed without complications; study ongoing 36