Synthetic Biologics Reports First Quarter 2019 Operational Highlights and Financial Results -- Enrollment is Ongoing in Phase 2b Investigator-Sponsored Clinical Study of SYN-010, for the Treatment of IBS-C; Topline Data

Synthetic Biologics Reports First Quarter

2019 Operational Highlights and Financial Results

-- Enrollment is Ongoing in Phase

2b Investigator-Sponsored Clinical Study of SYN-010,

for the Treatment of IBS-C; Topline

Data Readout Anticipated in 2H 2019 --

-- Conference Call Today, May 8, 2019,

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

MD, May 8, 2019 - 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

"During the first quarter, we remained

keenly focused on executing our strategy to advance our portfolio of GI and microbiome-focused clinical programs," stated

Steven A. Shallcross, Chief Executive & Financial Officer of Synthetic Biologics. "Enrollment into our investigator-sponsored

Phase 2b clinical trial of SYN-010 in breath methane positive irritable bowel syndrome with constipation (IBS-C) patients is ongoing

and we believe we are on track to announce a topline data-readout during the second half of 2019. Importantly, we believe dose-response

data derived from this clinical study will further fortify the already well-established clinical data set for SYN-010 and enable

regulatory discussions to potentially simplify future registration studies."

Mr. Shallcross continued, "During the first quarter, we continued to pursue our strategy for a potential

secondary clinical indication for SYN-004 (ribaxamase) in a specialized patient population which may allow for more narrow and

cost-efficient clinical development opportunities. Discussions with key opinion leaders are progressing to determine an optimal

development pathway for SYN-004 to reduce the incidence and/or severity of adverse outcomes in allogeneic hematopoietic cell transplant

(HCT) recipients. These adverse outcomes include acute graft-versus-host-disease (aGVHD) and colonization of opportunistic pathogens

such as vancomycin resistant Enterococci (VRE), each of which are associated with poor outcomes and increased mortality in allogeneic

HCT recipients. We believe SYN-004 may provide a distinct benefit to patient outcomes in allogeneic HCT recipients, where gut microbiome

damage resulting from prolonged beta-lactam antibiotic use has been linked to the development of aGVHD and colonization by VRE.

We look forward to sharing important updates and progress for this potential indication."

Clinical Development and Operational

Quarter Ended March 31, 2019 Financial

General and administrative expenses decreased

by 30% to $1.1 million for the three months ended March 31, 2019, from $1.6 million for the three months ended March 31, 2018.

This decrease is primarily due to decreased stock-based compensation expense related to forfeitures and decreased share price,

along with the reduction of investor relations, registration, and legal costs. The charge related to stock-based compensation expense

was $65,000 for the three months ended March 31, 2019, compared to $349,000 the three months ended March 31, 2018.

Research and development expenses decreased

by 28% to $2.4 million for the three months ended March 31, 2019, from $3.4 million for the three months ended March 31, 2018.

This decrease is primarily the result of lower SYN-004 (ribaxamase) indirect program costs for the three months ended March 31,

2019, including salary and related expense reductions resulting from the 2018 restructuring and the fact that no clinical trial

activity for SYN-004 (ribaxamase) was ongoing during the quarter, offset by an increase in manufacturing costs for SYN-020. The

research and development costs incurred during the quarter were primarily related to the investigator-sponsored Phase 2b clinical

study of SYN-010. We anticipate research and development expense to increase in association with the ongoing Phase 2b investigator-sponsored

clinical study of SYN-010, a potential Phase 1/2 clinical trial of SYN-004 (ribaxamase) in allogeneic HCT recipients, and the continued

development of SYN-020. There was no charge related to stock-based compensation expense for the three months ended March 31, 2019

resulting from the 2018 restructuring, compared to $326,000 for the three months ended March 31, 2018.

Other income was $44,000 for the three

months ended March 31, 2019, compared to other income of $2.7 million for the three months ended March 31, 2018. Other income

for the three months ended March 31, 2019 is primarily comprised of interest income while the three months ended March 31, 3018

is 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.

Cash and cash equivalents as of March 31,

2019 was $24.7 million, a decrease of $4.2 million from December 31, 2018.

Synthetic Biologics will hold a conference

call today, Wednesday, May 8, 2019,

at 4:30 p.m. (ET). 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/1096/30393. An archive of the call will be available

for replay at the same URL, https://www.webcaster4.com/Webcast/Page/1096/30393, for

90 days after the call.

About Synthetic Biologics, Inc.

Synthetic Biologics, Inc. (NYSE American:

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

The Company's lead candidates are: (1) SYN-004 (ribaxamase) which is designed to degrade certain commonly used intravenous (IV)

beta-lactam antibiotics within the gastrointestinal (GI) tract to prevent microbiome damage, C. 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, 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

is also advancing SYN-020, an oral formulation of the enzyme intestinal alkaline phosphatase (IAP) to treat both local GI and

systemic diseases, and has completed proof-of-concept studies with monoclonal antibody therapies for the prevention and treatment

of pertussis. 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 being

on track to announce a topline data- readout during the second half of 2019, dose-response data derived from the investigator-sponsored

Phase 2b clinical trial of SYN-010 further fortifying the already well-established clinical data set for SYN-010 and enabling regulatory

discussions to potentially simplify future registration studies, a potential secondary clinical indication for SYN-004 (ribaxamase)

in a specialized patient population which may allow for more narrow and cost-efficient clinical development opportunities, SYN-004

providing a distinct benefit to patient outcomes in allogeneic HCT recipients, initiation of a Phase 1/2 clinical study in allogeneic

HCT patients in 2H 2019, filing of a US IND application in Q4 2019 for SYN-020, commencing a Phase 1 clinical trial for SYN-020

in Q1 2020, and extending Synthetic Biologics' projected cash runway through at least the end of Q2 2020. 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 Synthetic Biologics'

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, SYN-010 and SYN-020 to be commenced or completed on time or to achieve

desired results and benefits, a failure to file INDs when anticipated, 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, 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 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, 2018 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