Synthetic Biologics Reports First Quarter 2018 Operational Highlights and Financial Results - Preliminary Agreement Reached with FDA on Proposed Phase 3 Clinical Trial Synopsis for SYN-004 (ribaxamase) - - Conference Cal

Synthetic Biologics Reports First Quarter

2018 Operational Highlights and Financial Results

- Preliminary Agreement Reached with

FDA on Proposed Phase 3 Clinical Trial

Synopsis for SYN-004 (ribaxamase)

- Conference Call Today, May 8, 2018,

at 4:30 p.m. (EDT) -

MD, May 8, 2018 - Synthetic Biologics, Inc. (NYSE American: SYN), a late-stage

clinical company developing therapeutics that preserve the microbiome to protect and restore the health of patients, today

provided an operational update and reported financial results for the three months ended March 31, 2018.

"During the first quarter, we held

several collaborative and productive meetings with the U.S. Food and Drug Administration (FDA) and have reached preliminary agreement

on a clinical trial synopsis for our planned Phase 3 clinical program for ribaxamase, our first-in-class therapeutic intervention

designed to prevent the onset of primary C. difficile infection (CDI) by protecting the gut microbiome from antibiotic-mediated

dysbiosis," stated Steven A. Shallcross, Interim Chief Executive Officer and Chief Financial Officer.

"The establishment of key components

of our planned Phase 3 clinical trial, including the important distinction of evaluating efficacy and safety as separate, co-primary

endpoints, is a significant milestone in ribaxamase's clinical development. Looking ahead, we plan to continue to work closely

with the FDA to define the remaining elements of our trial protocol and intend to share these outcomes following the completion

of our End-of-Phase 2 meeting, expected during the second half of 2018. Antibiotic exposure is the major risk factor for CDI, and

there are more than 450,000 cases of CDI each year in the U.S. If approved, ribaxamase will be the first intervention specifically

designed to prevent CDI associated with the most commonly used IV antibiotics," concluded Shallcross.

Clinical Development and Operational

Quarter Ended March 31, 2018 Financial

General and administrative expenses decreased

to $1.6 million for the three months ended March 31, 2018, compared to $2.1 million for the same period in 2017. This decrease

is primarily the result of lower salary expense, stock compensation, and related benefits costs incurred in 2018 due to the resignation

of the Chief Executive Officer, along with the reduction of travel, consulting fees, and legal expenses in 2018 offset by higher

registration fees. The charge related to stock-based compensation expense was $349,000 for the three months ended March 31, 2018,

compared to $698,000 for the same period in 2017.

Research and development expenses decreased

to $3.4 million for the three months ended March 31, 2018, compared to $6.1 million for the same period in 2017. This decrease

is primarily the result of lower SYN-004 (ribaxamase) and SYN-010 program costs for 2018 since no clinical trials were ongoing

during the quarter. The research and development costs incurred during the quarter were primarily related to planning for future

Phase 3 (SYN-004) and Phase 2b/3 (SYN-010) clinical programs as we seek to secure the financial resources necessary for the completion

of these clinical trials. The charge related to stock-based compensation expense was $326,000 for the three months ended March

31, 2018, compared to $437,000 for the same period in 2017.

Other income was $2.7 million for the three

months ended March 31, 2018, compared to other income of $5.1 million for the same period in 2017. Other income for the three months

ended March 31, 2018 is primarily comprised of non-cash income of $2.7 million from the change in fair value of warrants. The decrease

in the fair value of the warrants was due to the decrease in our stock price from the prior quarter.

Cash and cash equivalents as of March 31,

2018 was $11.0 million, a decrease of $6.1 million from December 31, 2017.

Synthetic Biologics will hold a conference

call today, Tuesday, May 8, 2018,

at 4:30 p.m. (EDT). The dial-in information for the call is as follows, U.S. toll free: +1 888-347-5280 or International:

+1 412-902-4280. Participants are asked to dial in 15 minutes before the start of the call to register. The call will also be webcast

over the Internet at https://www.webcaster4.com/Webcast/Page/1096/25579. An archive of the call will be available for replay at

the same URL, https://www.webcaster4.com/Webcast/Page/1096/25579, for 90 days after the call.

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.

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 our plan to continue to work closely with the FDA to define the remaining elements of the Phase

3 clinical trial protocol, expectations that the Phase 3 clinical trial for SYN-004 will include two separate and decoupled 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 in addition to 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 compared

to placebo and evaluate mortality risk as the primary safety endpoint, which will be separate from the primary efficacy endpoint;

ribaxamase if approved being the first intervention specifically designed to prevent CDI associated with the most commonly used

IV antibiotics; 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 held during the second half of 2018; 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' ability to achieve acceptance of its product candidates in

the marketplace and the successful development, marketing or sale of Synthetic Biologics' products by competitors that render

Synthetic Biologics' products obsolete or non-competitive, the continued maintenance and growth of Synthetic Biologics' patent

estate, Synthetic Biologics becoming and remaining profitable, Synthetic Biologics' ability to obtain or maintain the capital

or grants necessary to fund its research and development activities, a loss of any of Synthetic Biologics' key scientists or management

personnel and other factors described in Synthetic Biologics' Form 10-K for the year ended December 31, 2017 and its other filings

with the SEC, including subsequent periodic reports on Forms 10-Q and 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