Exhibit 99.1 SELB AAV8 Empty Capsid plus ImmTOR™ (SEL-399) Human Proof-Of-Concept Study* T op Line Data Report of a Phase 1 Clinical Study in Healthy Volunteers November 8, 2021 * Joint study with AskBio
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The Ability to Inhibit The Development of AAV-Specific Antibodies Has The Potential to Enable Re-Dosing of AAV Gene Therapy • Adeno-associated virus (AAV) vectors are the most widely used in-vivo gene therapy vectors • AAV is non-self replicating and over time transgene expression may wane, particularly in growing tissues such as liver in childhood, suggesting that re-dosing of gene therapy may be necessary • Gene therapy with AAV induces a robust immune response in recipients, resulting in high level anti-AAV neutralizing antibodies (NAb) which precludes re-dosing of the gene therapy • Because NAb can reduce the efficiency of gene therapy, treatment with AAV gene therapy is restricted to patients with low levels of pre-existing NAb. For example, Zolgensma NAb titers must be ≤1:50 to be eligible for treatment • In addition to induction of anti-AAV NAb, the immune response to AAV gene therapy may be involved in observed gene therapy associated toxicities such as hepatotoxicity and thrombotic microvascular angiopathy • As a proof-of-concept study, we studied the ability of ImmTOR, an immune tolerizing drug, to inhibit anti-AAV8 NAb formation in healthy volunteers to evaluate the potential of ImmTOR for gene therapy re-dosing 3
AAV Gene Therapy Induces an Immune Response That Precludes Redosing, Inhibits Efficacy, And Causes Adverse Events • Lower efficacy • Prevention of re-dosing • Adverse events • Loss of efficacy 4
Potential for ImmTOR to Enhance AAV Gene Therapy Potential to enable re-dosing of AAV gene therapy • Prevention of AAV antibody production • Potential to re-dose • Inhibition of liver inflammation • Enhanced efficacy • More durable efficacy • Improved safety 5
Long-Term Inhibition of Anti-AAV8 Antibodies With 3 Monthly Doses of ImmTOR in Animals Single And Multiple Dose ImmTOR Data AAV-EC + AAV-EC + 1 Mice AAV8 Empty capsid 2e13 vp/kg Single dose ImmTOR 3 monthly doses ImmTOR 2.0 2.0 1.5 1.5 1.5 1.0 1.0 1.0 0.5 0.5 0.5 0.0 0.0 0.0 Day Day Day BL = baseline AAV-SEAP + AAV-SEAP + Nonhuman AAV8-SEAP 2e12 vg/kg Single dose ImmTOR 3 monthly doses ImmTOR 1.5 1.5 1.5 2 primates 1.0 1.0 1.0 0.5 0.5 0.5 0.0 0.0 0.0 BL 7 14 28 56 84 BL 7 14 28 56 84 BL 7 14 28 56 84 Day Day Day BL = baseline 1 2 ESGCT 2021 Poster P010 / ESGCT 2021 Poster 003 6 BL 7 12 19 27 42 55 70 84 112 140 168 BL 7 12 19 27 42 55 70 84 112 140 168 BL 7 12 19 27 42 55 70 84 112 140 168 Anti-AAV8 IgG (OD ) 450-570 Anti-AAV8 IgG OD 450-570
Study Design 7
SEL-399 Phase 1 Dose-Escalation Study: Objectives and Endpoints § General Objectives § To determine the immune response to AAV8 empty capsids (EMC-101), which contain no DNA payload, in healthy volunteers § To determine whether single escalating doses of ImmTOR can reduce the immune response to AAV8 empty capsids in healthy volunteers 8
SEL-399 Phase 1 Dose-Escalation Study: Subjects and Design § Total healthy volunteers enrolled: 23 (14 males and 9 females) § All subjects with anti-AAV8 NAb titers <1:5 at baseline § Randomized, placebo controlled and double-blind study Drug Infusions 30 45 60 75 90 Study Day 0 15 Placebo* + Empty Capsid (2E12 vp**/kg) n=3 Placebo + Empty Capsid (2E12 vp/kg) n=3 ImmTOR (0.15 mg/kg) + Empty Capsid (2E12 vp/kg) n=9 Placebo + Empty Capsid (2E12 vp/kg) n=2 ImmTOR (0.3 mg/kg) + Empty Capsid (2E12 vp/kg) n=6 Neutralizing antibodies (NAb) measurement * Placebo=saline infusion ** vp=viral particles 9
SEL-399 Phase 1 Dose- Escalation Study 10
Single Dose ImmTOR Inhibited Formation of Anti-AAV8 NAb at Day 30 100% of subjects dosed with 0.3 mg/kg ImmTOR had NAb titers ≤1:25 at Day 30 67% of subjects dosed with 0.3 mg/kg ImmTOR had NAb titers ≤1:5 at Day 30 Linear Scale Log Scale 10 2000 2000 1000 1500 100 1000 Titer 1:25 10 500 Titer 1:5 1 0 Empty Capsid (n=8) Empty Capsid + 0.15 mg/kg ImmTOR (n=9) Empty Capsid + 0.3 mg/kg ImmTOR (n=6) 11 NAb Titers Log NAb Titers 10
Single Dose ImmTOR Inhibited Formation of Median Anti-AAV8 NAb in a Dose- Dependent Manner at Day 30 1:5 Median NAb titers in subjects dosed with 0.3 mg/kg ImmTOR at Day 30 250-fold lower median NAb titers in subjects dosed with 0.3 mg/kg ImmTOR at Day 30 100000 Median 10000 Titer 1:1250 1000 50-Fold 250-Fold Difference 100 Difference 1:25 10 1:5 1 0 10 20 30 Days Empty Capsid (n=8) Empty Capsid + 0.15 mg/kg ImmTOR (n=9) Empty Capsid + 0.3 mg/kg ImmTOR (n=6) 12 Median Log NAb Titers 10
Subjects Treated With a Single Dose of ImmTOR Developed Delayed NAb Formation by Day 90 Two additional doses of ImmTOR may be required to maintain control beyond Day 30 2 of 6 subjects dosed with 0.3 mg/kg ImmTOR had NAb titers ≤1:25 at Day 90 1 of 6 subjects dosed with 0.3 mg/kg ImmTOR had NAb titers ≤1:5 at Day 90 Day 90 Day 30 Titer 1:25 Titer 1:5 Empty Capsid (n=8) Empty Capsid + 0.15 mg/kg ImmTOR (n=9) Empty Capsid + 0.3 mg/kg ImmTOR (n=6) 13
Empty Capsid Data In-Line With Single Dose ImmTOR NHP Data at Day 90 1 Human Nonhuman Primate Target For Redosing Single dose Single Dose Three Doses 1000000 100000 Titers too high for re-dosing 100000 10000 200+ 10000 Failure 1000 1000 Antibody levels may interfere 50-200 Intermediate with efficient re-dosing** 100 100 Patients eligible for re-dosing* 10 1-50 10 (Zolgensma eligibility: ≤1:50) Target 1 1 AAV8-SEAP AAV8-SEAP * May be dependent on gene therapy dose AAV8-EC + + + ** Ancillary approaches such as IgG protease ImmTOR ImmTOR ImmTOR pre-treatment may be required for re-dosing D0 D0, 28, 56 D0 1 14 ESGCT 2021 Poster 003 Log NAb Titers 10 Log NAb Titers 10
Safety All treatment-related adverse events were expected for ImmTOR, readily monitorable, and transient § There were no Serious Adverse Events (SAEs) § Adverse events related to ImmTOR were expected based on previous clinical trials § Stomatitis (mouth ulcers, redness, or pain) was most common § 3 of 9 subjects in 0.15 mg/kg group and 6 of 6 subjects in 0.3 mg/kg group, all mild to moderate § Average start of stomatitis was on day 11 with an average duration of 8 days § Symptoms were ameliorated with steroid mouth wash § Rash was next most common § 3 of 9 subjects in 0.15 mg/kg group and 3 of 6 subjects in 0.3 mg/kg group, all mild to moderate § Average start of rash was day 12 and resolved after an average of 23 days § No therapy was required § Asymptomatic and transient laboratory AEs in subjects receiving ImmTOR were seen in 2 subjects with mild to moderate thrombocytopenia and 1 subject with grade 3 hypertriglyceridemia 15
Summary 16
Summary and Conclusions § AAV8 empty capsids elicited a strong immune response with peak median anti-AAV8 NAb titers of 1:6875 § ImmTOR inhibited the formation of anti-AAV8 NAb in a dose-dependent manner at Day 30 § After Day 30, 2 of 6 subjects treated with 0.3 mg/kg ImmTOR maintained NAb titers ≤ 1:25, while remaining ImmTOR- treated subjects showed delayed formation of NAb reaching control levels by Day 90 § Animal studies suggest that, if NAb are inhibited at Day 30, administration of two additional monthly doses of ImmTOR may maintain control of NAb beyond 90 days § Safety findings included AEs previously observed with ImmTOR § This promising study in healthy volunteers provides support for the potential use of ImmTOR for the inhibition of neutralizing antibodies to AAV8 in gene therapy clinical trials 17
Q&A 18