Synthetic Biologics Reports First Quarter

Synthetic Biologics Reports First Quarter

2017 Operational Highlights and Financial Results

- Results from Phase 2b Proof-of-Concept

Study Exploratory Endpoints Demonstrate Ribaxamase Protects

and Preserves Gut Microbiome from

Ceftriaxone-mediated Dysbiosis -

-- Two Ribaxamase and Two

SYN-010 Presentations to be Included at

Digestive Disease Week (DDW

- Conference Call Today, May

4, 2017, at 4:30 p.m. (EDT) -

MD, May 4, 2017 - Synthetic Biologics, Inc. (NYSE MKT: 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, 2017.

"During the first quarter of 2017,

we announced several important clinical milestones which continue to drive momentum for Synthetic Biologics," said Jeffrey

Riley, President and Chief Executive Officer. "We announced positive results from our Phase 2b proof-of-concept clinical

trial for ribaxamase demonstrating the achievement of the primary endpoint of significantly reducing the incidence of primary Clostridium

difficile infection (CDI). In addition, data from this study demonstrated that ribaxamase significantly lowered the incidence

of new colonization by vancomycin-resistant enterococci (VRE), compared to placebo. Most recently, we shared supportive data from

exploratory endpoints in this trial with the CDC demonstrating ribaxamase successfully protected and preserved the naturally occurring

composition of the gut microbiome from the dysbiotic effects of IV ceftriaxone, compared to placebo. These results position ribaxamase

as a leader in clinical development for microbiome-based interventions specifically designed to prevent the incidence of primary

CDI and antimicrobial resistance (AMR) amongst at-risk patients. We continue to analyze data from this study and expect to share

results in the coming months from several additional exploratory endpoints designed to evaluate ribaxamase's ability to prevent

the emergence and proliferation of AMR in the gut microbiome."

Mr. Riley continued, "Since the

beginning of the first quarter, we also shared positive preclinical findings for SYN-005, our orphan drug program designed to

treat and prevent pertussis, which if translated to humans may provide a much-needed approach to protecting and treating the more

than 50 million global at-risk individuals, especially newborns in the developing world. Looking ahead, we are focused on building

upon the progress of the first quarter during the remainder of 2017. With the foundation of our Phase 2b/3 adaptive design pivotal

trial in place for SYN-010, intended to treat an underlying cause of IBS-C, we remain focused on solidifying the infrastructure

to support its continued successful clinical advancement."

Microbiome-Focused Clinical Program

SYN-004 - Prevention of C.

difficile infection (CDI), antibiotic-associated diarrhea (AAD) and the emergence of antimicrobial resistance (AMR):

SYN-010 - Treatment of irritable

bowel syndrome with constipation (IBS-C):

SYN-005 - Treatment and Prevention

of Pertussis (whooping cough)

Synthetic Biologics also announced that

four abstracts have been accepted for presentation at Digestive Disease Week 2017. Members of the Synthetic Biologics

team will present two posters on SYN-010 and two talks on ribaxamase at DDW .

Digestive Disease Week

Quarter Ended March 31, 2017 Financial

General and administrative expenses were

$2.1 million for the three months ended March 31, 2017, compared to $2.4 million for the same period in 2016. This decrease is

primarily the result of lower employee salary expense and related benefits costs along with reduced travel and legal expenses.

The charge related to stock-based compensation expense was $698,000 for the three months ended March 31, 2017, compared to $643,000

the same period in 2016.

Research and development expenses were

$6.0 million for the three months ended March 31, 2017, compared to $8.1 million for the same period in 2016. This decrease is

primarily the result of lower ribaxamase program costs associated with its clinical development program, as well as manufacturing

and research activities within our other microbiome-focused research and development activities. The charge related to non-cash

stock-based compensation expense was $437,000 for the three months ended March 31, 2017, compared to $409,000 for the same period

Other income was $5.1 million for the three

months ended March 31, 2017, compared to other expense of $0.5 million for the same period in 2016. Other income for the three

months ended March 31, 2017 is due to non-cash income of $5.1 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,

2017 was $13.5 million, a decrease of $5.6 million from December 31, 2017.

Synthetic Biologics will hold a conference

call today, Thursday, May 4, 2017, 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/1096/20882. An archive

of the call will be available for replay at the same URL, https://www.webcaster4.com/Webcast/Page/1096/20882, for 90 days

About Synthetic Biologics, Inc.

Synthetic Biologics, Inc. (NYSE MKT: SYN)

is a late-stage clinical company developing therapeutics designed to preserve the microbiome to protect and restore the health

of patients. The Company's lead candidates poised for Phase 3 development are: (1) 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), and (2) 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), antibiotic-associated diarrhea

(AAD) and the emergence of antimicrobial resistance (AMR). The Company is also developing preclinical stage 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 press 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 results of Synthetic Biologics' Phase 2b proof-of-concept clinical trial for ribaxamase

positioning ribaxamase as a leader in clinical development for microbiome-based interventions specifically designed to prevent

the incidence of primary CDI and antimicrobial resistance (AMR) amongst at-risk patients, the continued analysis of data from this

study, the expected sharing of results in the coming months from several additional exploratory endpoints designed to evaluate

ribaxamase's ability to prevent the emergence and proliferation of AMR in the gut microbiome, the ability of SYN-005 to provide

an approach to protecting and treating the more than 50 million global at-risk individuals for pertussis, the size of the pertussis

market, the continued clinical advancement of SYN-010 for the treatment of IBS-C, the anticipated requesting end of Phase 2 meeting

with FDA , expected initiation of Phase 3 clinical trials for SYN-004 and Phase2b/3 clinical trial for SYN-010 and the timing of

the initiation, 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' product candidates demonstrating safety and effectiveness,

as well as results that are consistent with prior results, Synthetic Biologics' ability to initiate clinical trials and if initiated,

to complete them on time and achieve desired results and benefits, Synthetic Biologics' ability to successfully design a protocol

and a corresponding statistical analysis plan to support the execution of its pivotal clinical trials, Synthetic Biologics' clinical

trials continuing enrollment as expected, Synthetic Biologics' ability to obtain regulatory approvals for commercialization of

product candidates or to comply with ongoing regulatory requirements, regulatory limitations relating to Synthetic Biologics' ability

to promote or commercialize its product candidates for specific indications, 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'