Synthetic Biologics Announces Positive Outcome of Data and Safety Monitoring Committee (DSMC) Review in Phase 1b/2a Clinical Trial of SYN-004 (ribaxamase) in Allogeneic Hematopoietic Cell Transplant Recipients - The DSMC

Synthetic Biologics Announces Positive Outcome

of Data and Safety Monitoring Committee (DSMC) Review in Phase 1b/2a Clinical Trial of SYN-004 (ribaxamase) in Allogeneic Hematopoietic

Cell Transplant Recipients

- The DSMC has reviewed the safety and pharmacokinetic

data from Cohort 1 and recommended that the study may proceed to enroll patients into Cohort 2 -

MD, September 27, 2022 - Synthetic Biologics, Inc. (NYSE American: SYN), a diversified

clinical-stage company developing therapeutics designed to treat diseases in areas of high unmet need, today

announced a positive outcome from the Data and Safety Monitoring Committee (DSMC) review of results from the first Cohort of its Phase

1b/2a randomized, double-blinded, placebo-controlled clinical trial of SYN-004 (ribaxamase) in

allogeneic hematopoietic cell transplant (HCT) recipients for the prevention of acute graft-versus-host-disease (aGVHD).

Cohort 1 enrolled 19 patients who received at

least 1 dose of study drug (SYN-004 or Placebo randomized 2:1). Sixteen patients received at least one dose of intravenous (IV) meropenem

and 12 of these patients completed sufficient doses of IV meropenem to be evaluable towards the study endpoints. The study is on-going

and remains blinded; however, key findings from blinded data for Cohort 1 are included below:

Based on a review of the safety and pharmacokinetic

data, the DSMC has recommended that the study may proceed to enroll Cohort 2 in which study drug (SYN-004 or Placebo) will be administered

in combination with the IV beta-lactam antibiotic piperacillin/tazobactam.

Steven A. Shallcross, Chief Executive Officer

of Synthetic Biologics, commented, "These encouraging data support the clinical advancement of SYN-004 and build on the growing

data that underscore its therapeutic potential. We remain on track to initiate the second cohort in Q4 2022, which is designed to evaluate

the combination of SYN-004 with piperacillin/tazobactam. We are very grateful for the tremendous support from Dr. Dubberke and his team

at Washington University as we take critical steps with the goal of improving standard treatment for these highly susceptible patients

by overcoming existing limitations of broad-spectrum IV beta-lactam antibiotics."

About the Phase 1b/2a Clinical Trial

The ongoing randomized, double-blinded, placebo-controlled

Phase 1b/2a clinical trial is being conducted at Washington University School of Medicine in St. Louis. The trial is designed to evaluate

the safety, tolerability, and potential absorption of oral SYN-004 (150 mg q.i.d. for a maximum of 28 days) into the systemic circulation

of allogeneic HCT recipients who receive an IV antibiotic to treat fever. To mitigate risk, Cohort 1 of the study administered meropenem

as the study-assigned antibiotic. Meropenem is a carbapenem antibiotic that is not metabolized by SYN-004. Patients in Cohorts 2 and

3 will be administered piperacillin/tazobactam and cefepime respectively, each of which can be metabolized by SYN-004. The trial is also

designed to evaluate potential protective effects of SYN-004 on the gut microbiome as well as generate preliminary information on potential

therapeutic benefits and patient outcomes of SYN-004 in allogeneic HCT recipients. The trial is expected to enroll up to 36 participants

with three sequential cohorts, each evaluating a different study-assigned IV beta-lactam antibiotic. Safety and pharmacokinetic data

for each cohort will be reviewed by an independent Data and Safety Monitoring Committee that will make a recommendation on whether to

proceed to the next IV beta-lactam antibiotic. More information on the study is available here (NCT04692181).

About SYN-004 (ribaxamase)

SYN-004 (ribaxamase) is an oral prophylactic therapy designed to degrade

certain IV beta-lactam antibiotics within the GI tract and maintain the natural balance of the gut microbiome for the prevention of Clostridioides

difficile infection (CDI), overgrowth of pathogenic organisms, the emergence of antimicrobial resistance (AMR) and acute graft-versus-host-disease

(aGVHD) in allogeneic hematopoietic cell transplant (HCT) recipients. Allogeneic HCT recipients routinely receive long courses of IV

beta-lactam antibiotics to treat infection following conditioning therapy. Antibiotic-mediated damage of the gut microbiome in allogeneic

HCT recipients may lead to adverse outcomes including CDI, VRE colonization and potentially fatal bacteremia and aGVHD. A previously

completed placebo-controlled Phase 2b clinical trial of 412 patients demonstrated SYN-004 protected the gut microbiome from antibiotic-mediated

dysbiosis. Patients who received SYN-004 also demonstrated significantly better maintenance and recovery of the gut microbiome as well

as lower incidences of new colonization by opportunistic and potentially pathogenic microorganisms such as VRE.

About Synthetic Biologics, Inc.

Synthetic Biologics, Inc. (NYSE American: SYN) is a diversified clinical-stage

company developing therapeutics designed to treat diseases in areas of high unmet need. The Company advanced into oncology through the

acquisition of VCN Biosciences, S.L. (VCN), who have developed a new oncolytic adenovirus platform designed for intravenous (IV), intravitreal

and antitumoral delivery to trigger tumor cell death, improve access of co-administered cancer therapies to the tumor, and promote a robust

and sustained anti-tumor response by the patient's immune system. The Company's lead candidates are: (1) VCN-01, an oncolytic adenovirus

designed to replicate selectively and aggressively within tumor cells, and to degrade the tumor stroma barrier that serves as a significant

physical and immunosuppressive barrier to cancer treatment; (2) SYN-004 (ribaxamase) which is designed to degrade certain commonly used

IV beta-lactam antibiotics within the gastrointestinal (GI) tract to prevent (a) microbiome damage, (b) Clostridioides difficile infection

(CDI), (c) overgrowth of pathogenic organisms, (d) the emergence of antimicrobial resistance (AMR), and (e) acute graft-versus-host-disease

(aGVHD) in allogeneic hematopoietic cell transplant (HCT) recipients, and (3) SYN-020, a recombinant oral formulation of the enzyme intestinal

alkaline phosphatase (IAP) produced under cGMP conditions and intended to treat both local GI and systemic diseases. For more information,

please visit Synthetic Biologics' website at www.syntheticbiologics.com.

Forward-Looking Statement

This release contains forward-looking statements within the meaning

of the Private Securities Litigation Reform Act of 1995. In some cases forward-looking statements can be identified by terminology such

as "may," "should," "potential," "continue," "expects," "anticipates," "intends,"

"plans," "believes," "estimates," and similar expressions, and include statements regarding the encouraging

data supporting the clinical advancement of SYN-004 and building on the growing data that underscore its therapeutic potential, remaining

on track to initiate the second cohort in Q4 2022, which is designed to evaluate the combination of SYN-004 with piperacillin/tazobactam,

SYN-004 improving standard treatment for the highly susceptible patients by overcoming existing limitations of broad-spectrum IV beta-lactam

antibiotics, the potential protective effects of SYN-004 on the gut microbiome and generating preliminary information on potential therapeutic

benefits and patient outcomes of SYN-004 in allogeneic HCT recipients and the trial enrolling up to 36 participants with three sequential

cohorts, each evaluating a different study-assigned IV beta-lactam antibiotic. These forward-looking statements are based on management's

expectations and assumptions as of the date of this press release and are subject to a number of risks and uncertainties, many of which

are difficult to predict that could cause actual results to differ materially from current expectations and assumptions from those set

forth or implied by any forward-looking statements. Important factors that could cause actual results to differ materially from current

expectations include, among others, the ability of SYN-004 to improve standard treatment for the highly susceptible patients by overcoming

existing limitations of broad-spectrum IV beta-lactam antibiotics, the ability to enroll the anticipated number of patients in the Phase

1b/2a clinical trial, Synthetic Biologics' ability to reach clinical milestones when anticipated, Synthetic Biologics' ability

to successfully combine and operate the business of Synthetic Biologics and VCN, Synthetic Biologics' and VCN's product candidates demonstrating

safety and effectiveness, as well as results that are consistent with prior results; the ability to complete clinical trials on time and

achieve the desired results and benefits, continuing clinical trial enrollment as expected; the ability to obtain regulatory approval

for commercialization of product candidates or to comply with ongoing regulatory requirements, regulatory limitations relating to Synthetic

Biologics' and VCN's ability to promote or commercialize their product candidates for the specific indications, acceptance of product

candidates in the marketplace and the successful development, marketing or sale of Synthetic Biologics' and VCN's products, developments

by competitors that render such products obsolete or non-competitive, Synthetic Biologics' and VCN's ability to maintain license agreements,

the continued maintenance and growth of Synthetic Biologics' and VCN's patent estate, the ability to continue to remain well financed,

and other factors described in Synthetic Biologics' Annual Report on Form 10-K for the year ended December 31, 2021 and its other filings

with the SEC, including subsequent periodic reports on Forms 10-Q and current reports on Form 8-K. The information in this release is

provided only as of the date of this release, and Synthetic Biologics undertakes no obligation to update any forward-looking statements

contained in this release on account of new information, future events, or otherwise, except as required by law.

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LifeSci Advisors, LLC