Assembly Biosciences : Interim Results from Phase 1b Clinical Studies of Long-Acting Helicase-Primase Inhibitor Candidates ABI-1179 and ABI-5366 in Recurrent Genital Herpes

Interim Results from Phase 1b Clinical Studies of Long-Acting Helicase-Primase Inhibitor Candidates ABI-1179 and ABI-5366 in Recurrent Genital Herpes

December 8, 2025 Nasdaq: ASMB

©2025 ASSEMBLY BIOSCIENCES, INC.

• ABI-1179 weekly cohort B1 exceeds Phase 1b expectations with

  • 98% reduction in HSV-2 shedding (p<0.01) vs placebo

  • 92% reduction in virologically confirmed lesion rate (p<0.01) vs placebo

• ABI-5366 weekly cohort B1/B2 unblinded safety data released

1. Overall lesion rate presented in August 2025; updated today to virologically confirmed lesion rate

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• ABI-5366 monthly cohort B3 shows potent antiviral activity

  • 76% reduction in HSV-2 shedding (p<0.01) vs placebo

  • 88% reduction in virologically confirmed lesion rate (p=0.01) vs placebo

Results Released Today

ABI-5366 and ABI-1179 - Helicase-Primase Inhibitors for Recurrent Genital Herpes

Positive Interim Phase 1b Results for Two Promising Long-Acting Candidates

• ABI-5366 weekly cohort B2 exceeds Phase 1b expectations with

  • 94% reduction in HSV-2 shedding (p<0.01) vs placebo

  • 97% reduction in virologically confirmed lesion rate (p<0.05) vs placebo1

August 2025

ABI-1179-101 Phase 1b Study Design

• Double-blind, placebo-controlled sequential cohorts
• All participants seropositive for HSV-2 with recurrent genital herpes
• Each cohort with 20 patients receiving ABI-1179 and 5 patients receiving placebo

B1	Weekly	50mg
B2	Weekly	20mg
B3	Weekly	10 mg
B4	TBD	TBD

Cohort B1

(N=25)

Follow-up

period

D8

1

D35

Data in current analysis

COHORT

REGIMEN

WEEKLY DOSE

Follow-up period

Cohort B4

(N=25)

Follow-up period

Cohort B3

(N=25)

Cohort B2

(N=25)

Follow-up period

KEY EFFICACY ASSESSMENTS DATA IN CURRENT ANALYSIS

• Anogenital swabs (Day 8-35); e.g., viral shedding rate

• Daily diary of symptoms; e.g., days with lesions

• Diary data through D35

• 100% Shedding data

• Safety data through Day 57

Interim ABI-1179 Phase 1b data as of November 25, 2025

1. Dosing begins on Day 1 with swab and diary evaluations beginning on Day 8

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BASELINE DEMOGRAPHICS AND DISEASE CHARACTERISTICS	ABI-1179 20mg weekly/PBO (N=24)	ABI-1179 50mg weekly/PBO (N=25)
Age, median (range) 40 (24-61) 40 (23 - 60)

Male, N (%)	13 (54%)	7 (28%)
Race, N (%)
White	21 (88%)	23 (92%)
Black/African American	0	1 (4%)
Native Hawaiian/Pacific Islander	0	3 (12%)
Other	3 (13%)	2 (8%)
BMI, median (range)	25.4 (20.1 - 30.2)	26.7 (18.9 - 31.4)
Years since HSV Diagnosis, median (IQR)	6.9 (3.2 - 17.3)	8.0 (4.6 - 10.6)
Number of Lesions in past 12 months or prior to suppressive treatment, median (IQR)	5.5 (5.0 - 6.0)	6.0 (5.0 - 7.0)
Suppressive Treatment at Screening, N (%)	18 (75%)	20 (80%)

Interim ABI-1179 Phase 1b data as of November 25, 2025

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ABI-1179 Phase 1b:

Baseline Demographics and Disease Characteristics

PARAMETER	ABI-1179 20mg weekly/PBO N=24	ABI-1179 50mg weekly/ PBO N=25
Subjects with any Treatment Emergent Adverse Events (TEAE) (max 17 (71%) 23 (92%) grade), N (%)

Grade 1, N (%)	12 (50%)	9 (36%)
Grade 2, N (%)	5 (21%)	13 (52%)
Grade 3, N (%)	0	1 (4%)1
Grade 4, N (%)	0	0
TEAE Related to Study Drug, N (%)	8 (33%)	10 (40%)
TEAE Leading to Study Drug Discontinuation, N (%)	0	0
Serious Adverse Event	0	0
Death	0	0
Treatment Emergent Lab Abnormalities, N (%)	9 (38%)	8 (32%)
Grade 1, N (%)	8 (33%)	6 (24%)
Grade 2, N (%)	2 (8%)	3 (12%)
Grade 3, N (%)	0	0
Grade 4, N (%)	0	0

Safety data includes patients through Day 57

Interim ABI-1179 Phase 1b data as of November 25, 2025

1. Grade 3 migraine reported in 50mg/PBO by participant with a medical history of migraines

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ABI-1179 Phase 1b:

Safety Summary - Adverse Events Cohorts B1 & B2

ABI-1179 Phase 1b:

Cohorts B1 and B2 with Significant Reduction in HSV-2 Shedding

98%

reduction in HSV-2 shedding rate for cohort B12

20%

Percent Shedding

15%

10%

5%

0%

16.9%

p < 0.011

p < 0.011

1.4% 0.4%

Placebo N=9

Cohort B2 20mg Weekly N=20

Cohort B1 50mg Weekly N=20

• Significant reduction in HSV-2 shedding rate for Cohorts B1 and B2 compared to Placebo

  Interim ABI-1179 Phase 1b data as of November 25, 2025

  1. Poisson regression analysis; 2. Data are interim estimates based on pooled placebo patients from these cohorts and may change with additional cohorts

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  ABI-1179 Phase 1b:

  Reduction in HSV-2 High Viral Load Shedding in Cohorts B1 and B2

  >99%

  reduction in HSV-2 high viral load shedding for

  cohort B13,4

  16%

  Percent Swabs with >104 Copies/mL

  12%

  8%

  4%

  0%

  11.8%

  p < 0.011

  p4

  0.5% <0.1%4

  Placebo N=9

  Cohort B2 20mg Weekly N=20

  Cohort B1 50mg Weekly N=20

• Significant reduction in HSV-2 high viral load shedding for Cohorts B1 and B2 compared to Placebo

• Near complete elimination of HSV-2 high viral load swabs >104 copies/mL for both cohorts

  ‒ Shedding >104 copies/mL a surrogate for increased HSV-2 transmission2

  Interim ABI-1179 Phase 1b data as of November 25, 2025. 1. Poisson regression analysis; 2. Schiffer JT et al. J.R.Soc.Interface 11, 2014; 3. Data are interim estimates based on available placebo patients and may change with subsequent cohorts; 4. P-value cannot be reliably calculated given the >99% reduction compared to placebo. The observed difference is consistent with a highly significant effect.

  8

  
  
  
  
  

  ABI-1179 Phase 1b:

  Cohorts B1 and B2 with Significant Reduction in Virologically Confirmed Lesion Rate

  91%

  reduction in

  virologically confirmed2 lesion rate for cohort B13

  10%

  Percent Days with Lesions

  8%

  6%

  4%

  2%

  0%

  p4

  8.4%

  p < 0.011

  <0.1% 4 0.7%

  Placebo N=9

  Cohort B2 20mg Weekly N=20

  Cohort B1 50mg Weekly N=20

• Significant reduction in virologically confirmed lesion rate for Cohorts B1 and B2 compared to Placebo

Interim ABI-1179 Phase 1b data as of November 25, 2025 1. Poisson regression analysis; 2. Virologically confirmed lesions include lesions with any positive HSV-2 swab taken during the duration of the lesion; 3. Data are interim estimates based on pooled placebo patients from these

cohorts and may change with additional cohorts; 4. P-value cannot be reliably calculated given the >99% reduction compared to placebo. The observed difference is consistent with a highly significant effect.

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ABI-1179 PH1B STATUS UPDATE

• Two weekly dosing cohorts have completed treatment (B1 and B2)

• One weekly dosing cohort is currently enrolling (B3)

• Phase 2 enabling activities underway

  PHASE 1B TRIAL GOALS PHASE 1B COHORT B1 (50 MG, QW) RESULTS

  • 80 to 85% reduction in HSV-2 shedding vs. placebo

  • Significant reduction in high viral load swabs1

  • Directional reduction in genital lesions

  • Clean safety profile

    98% reduction in HSV-2 shedding (p<0.01)

    >99% reduction in high viral load swabs

    91% reduction in virologically confirmed2 genital lesions (p<0.01)

    No safety signals identified to date

    Interim ABI-1179 Phase 1b data as of November 25, 2025

    1. Surrogate for HSV-2 transmission; Schiffer JT et al. J.R.Soc.Interface 11, 2014; 2. Virologically confirmed lesions include lesions with any positive HSV-2 swab taken during the duration of the lesion. QW, weekly. 10

    
    
    

    Executive Summary:

    ABI-1179 Phase 1b Interim Update

    
    
    

    ABI-5366-101 Phase 1b Study Design

  • Double-blind, placebo-controlled sequential cohorts
  • All participants seropositive for HSV-2 with recurrent genital herpes
  • Each cohort with 20 patients receiving ABI-5366 and 5 patients receiving placebo

    D81

    D36

    D98

    COHORT REGIMEN LOADING DOSE

    WEEKLY DOSE

    Follow-up period

    COHORT B1

    (N=25)

COHORT B3

(N=25)

COHORT B2

(N=25)

Follow-up period

B1	Weekly	150 mg	30 mg
B2	Weekly	350 mg	350 mg
B3	Monthly	350 mg x5	-

Follow-up period

KEY EFFICACY ASSESSMENTS

• Anogenital swabs (Day 8-36); e.g., viral shedding rate

• Daily diary of symptoms; e.g., days with lesions

  DATA IN CURRENT ANALYSIS

• Diary data through D36

• 100% Shedding data

• Complete safety data for cohorts B1 and B2

• Safety data up to Day 43 for cohort B3

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Interim ABI-5366 Phase 1b data as of November 25, 2025. Interim ABI-5366 Phase 1b data for cohorts B1 and B2 as of July 29, 2025 was previously presented

1. Dosing begins on Day 1 with swab and diary evaluations beginning on Day 8