Synthetic Biologics Provides Update on Development of SYN-004 (ribaxamase), for the Prevention of C. difficile Infection Rockville, MD

Synthetic Biologics Provides Update

on Development of SYN-004 (ribaxamase),

for the Prevention of C. difficile

Rockville, MD, April 23, 2018

- Synthetic Biologics, Inc. (NYSE American: SYN), a late-stage clinical company developing therapeutics designed to

preserve the microbiome to protect and restore the health of patients, today announced that it has preliminary agreement from

the U.S. Food & Drug Administration (FDA), on a proposed clinical trial synopsis for its planned Phase 3 clinical trial

for SYN-004 (ribaxamase). In accordance with recommendations and guidance received from the FDA, the Company expects the

Phase 3 trial to include separate co-primary endpoints designed to evaluate the efficacy and safety of ribaxamase in a

patient population being treated with a representative selection of intravenous (IV) beta-lactam antibiotics.

The proposed Phase 3 study discussed with

the FDA comprises a global, event-driven clinical trial with a fixed maximum number of patients for total enrollment. The Phase

3 study proposes to evaluate the potential efficacy and safety of ribaxamase in a broader patient population by the inclusion of

additional IV beta-lactam antibiotics with ceftriaxone and by enrolling patients with a variety of underlying infections. Based

on a series of collaborative discussions with the FDA, the Company expects the primary efficacy endpoint of the proposed Phase

3 clinical trial will be the reduction of the incidence of Clostridium difficile infection (CDI) in the ribaxamase treatment

group relative to placebo.

The Company also announced it has preliminary

agreement from the FDA to evaluate mortality risk as the primary safety endpoint for the trial, which will be separate from the

primary efficacy endpoint of reduction of the incidence of CDI. The designation of efficacy and safety as separate and decoupled

endpoints is critical for clinical studies of this nature, where the underlying population is projected to have a comparatively

high incidence of safety events that may significantly dilute the smaller number of CDI events.

"Following recent collaborative discussions

with the FDA, we have gained clarity on several significant elements of the proposed Phase 3 clinical program, which we believe

provide a path forward to develop ribaxamase, a product which, if approved, may address the serious and unmet health impacts associated

with antibiotic-mediated CDI," stated Steven A. Shallcross, Interim Chief Executive Officer and Chief Financial Officer.

"We look forward to sharing the remaining elements of the proposed Phase 3 trial upon the completion of our End-of-Phase

2 meeting with the FDA during the second half of 2018, and anticipate initiating this trial during the second half of 2019."

Synthetic Biologics further announced that

during the development of the proposed Phase 3 clinical trial, the FDA undertook an additional review of data and analysis submitted

by the Company from the previously completed ribaxamase Phase 2b clinical trial. Following FDA's review of the additional

data, it was determined that the requirements for Breakthrough Therapy Designation were no longer met due to the numerical imbalance

in fatal adverse events observed in the study which could not be fully evaluated due to the limited safety database, and the study's

method of statistical treatment of patients who did not complete the study for any reason. The Company has reached agreement with

the FDA on how each of these factors will be addressed in the Phase 3 trial by evaluating safety and efficacy endpoints separately

as described above. As a result, and with the consent of the FDA, the Company has voluntarily withdrawn the Breakthrough Therapy

Designation for the ribaxamase program. The FDA stated in their official response to the Company that they remain committed to

working with Synthetic Biologics on the development of the ribaxamase program, and the withdrawal of Breakthrough Therapy Designation

will not affect interactions between the two parties.

"We are grateful to have received

guidance and continued support from the FDA as part of ribaxamase's development and remain encouraged that a robust, controlled,

and well-designed clinical trial may provide sufficient efficacy and safety data to support a pathway towards marketing approval

for ribaxamase," concluded Shallcross.

About Synthetic Biologics, Inc.

Synthetic Biologics, Inc. (NYSE American:

SYN) is a late-stage clinical company developing therapeutics that preserve the microbiome to protect and restore the health of

patients. The Company's lead candidates poised for Phase 3 development are: (1) SYN-004 (ribaxamase) which is designed to protect

the gut microbiome from the effects of certain commonly used intravenous (IV) beta-lactam antibiotics for the prevention of C.

difficile infection (CDI), overgrowth of pathogenic organisms and the emergence of antimicrobial resistance (AMR), and (2)

SYN-010 which is intended to reduce the impact of methane producing organisms in the gut microbiome to treat an underlying cause

of irritable bowel syndrome with constipation (IBS-C). The Company's preclinical pursuits include an oral formulation of

the enzyme intestinal alkaline phosphatase (IAP) to treat both local GI and systemic diseases as well as monoclonal antibody therapies

for the prevention and treatment of pertussis, and novel discovery stage biotherapeutics for the treatment of phenylketonuria

(PKU). For more information, please visit Synthetic Biologics' website at www.syntheticbiologics.com.

About Clostridium difficile infection

Clostridium difficile infection (CDI) is a leading hospital

acquired infection in the U.S., with more than 453,0001 patients diagnosed annually. CDI results in approximately 29,000

deaths1, $5.42 billion in additional healthcare costs, as well as significant and sometimes prolonged illness.

Approximately 1 in 5 CDI patients experience at least one CDI recurrence3.

1. Lessa, F.C., Winsto.,

& McDonald, L.C; (2015). Emerging Infections Program C. difficile Surveillance Team. Burden of Clostridium

difficile infection in the United States. New England Journal of Medicine. Retrieved from http://www.nejm.org/doi/full/10.1056/NEJMc1505190#t=article (Last accessed August 2017).

2. Desai K, Gupta SB,

Dubberke ER, Prabhu VS, Browne C, Mast TC. Epidemiological and economic burden of Clostridium difficile in the

United States: estimates from a modeling approach. BMC Infectious Diseases. 2016;16:303. doi:10.1186/s12879-016-1610-3.

al. "Excess length of stay and mortality due to Clostridium difficile infection: a multi-state modelling approach." The

Journal of hospital infection 88 4 (2014): 213-7. DOI: 10.1016/j.jhin.2014.08.008

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 includes statements regarding expectations that the Phase 3 clinical trial for SYN-004 will include separate co-primary endpoints,

comprise a global, event-driven clinical trial with a fixed maximum number of patients for total enrollment, evaluate the potential

efficacy and safety of ribaxamase in a broader patient population by the inclusion of additional IV beta-lactam antibiotics with

ceftriaxone and by enrolling patients with a variety of underlying infections, have as the primary efficacy endpoint the reduction

of the incidence of Clostridium difficile infection (CDI) in the ribaxamase treatment group relative to placebo and evaluate mortality

risk as the primary safety endpoint, which will be separate from the primary efficacy endpoint; the belief that the proposed Phase

3 clinical program will provide a path forward to develop ribaxamase which if approved, may address the serious and unmet health

impacts associated with antibiotic-mediated CDI; the anticipated timing of the initiation of the Phase 3 clinical trial during

the second half of 2019 following the end of Phase 2 meeting, which is anticipated to be during the second half of 2018; the belief

that a robust, controlled, and well-designed clinical trial may provide sufficient efficacy and safety data to support a pathway

towards marketing approval for ribaxamase; and the potential benefits of SYN-004 and SYN-010. 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, Synthetic Biologics' ability to design a Phase 3 trial with

the co-primary endpoints and receive FDA approval for such design; Synthetic Biologics' ability to implement the Phase 3 program

as a global, event-driven clinical trial, Synthetic Biologics' ability to initiate the Phase 3 clinical program in the second half

of 2019 following an end of Phase 2 meeting with the FDA during the second half of 2018, Synthetic Biologics' ability to establish

a path forward to develop ribaxamase and conduct a robust, controlled and well-designed clinical trial that may provide sufficient

efficacy and safety data to support a pathway towards marketing approval for ribaxamase, Synthetic Biologics' ability to regain

compliance with the continued listing standards of the NYSE American by September 2, 2019, Synthetic Biologics' ability to

comply with other continued listing requirements of the NYSE American, the ability of its product candidates to demonstrate safety

and effectiveness, as well as results that are consistent with prior results, Synthetic Biologics' clinical trials continuing enrollment

as expected, a failure to receive the necessary regulatory approvals for commercialization of Synthetic Biologics' therapeutics,

including approval of proposed trial designs, a failure of Synthetic Biologics' clinical trials, and those conducted by investigators,

for SYN-004 and SYN-010 to be commenced or completed on time or to achieve desired results and benefits, a failure of Synthetic

Biologics' clinical trials to continue enrollment as expected or receive anticipated funding, a failure of Synthetic Biologics

to successfully develop, market or sell its products, Synthetic Biologics' inability to maintain its material licensing agreements,

or a failure by Synthetic Biologics or its strategic partners to successfully commercialize products, Synthetic Biologics'