Synthetic Biologics Reports Second Quarter 2019 Operational Highlights and Financial Results -- Announced Clinical Trial Agreement with Washington University School of Medicine to Conduct a Phase 1b/2a Clinical Trial of

Synthetic Biologics Reports Second Quarter

2019 Operational Highlights and Financial Results

-- Announced Clinical Trial Agreement

with Washington University School of Medicine to Conduct a Phase 1b/2a Clinical Trial of SYN-004 (ribaxamase) in Allogeneic Hematopoietic

Cell Transplant Recipients --

-- Conference Call Today, August 8,

2019, at 4:30 p.m. (ET) --

MD, August 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

June 30, 2019. The Company also announced today a clinical trial agreement with Washington University School of Medicine in St.

Louis ("Washington University") to conduct a Phase 1b/2a clinical trial of SYN-004 (ribaxamase), with enrollment expected

to begin in the first quarter of 2020.

clinical trial collaboration with Washington University is an important step in our pursuit of a more cost-effective development

strategy for SYN-004 targeting a more specialized patient population," stated Steven A. Shallcross, Chief Executive and Financial

Officer. "SYN-004's unique mechanism of action designed to degrade intravenous (IV) beta-lactam antibiotics

and prevent dysbiosis of the gut microbiome has the potential to significantly improve outcomes for patients who undergo allogeneic

hematopoietic cell transplantation (HCT), an area of significant unmet medical need."

"Prompt initiation of broad-spectrum

IV beta-lactam antibiotics at onset of fever after conditioning chemotherapy for allogenic HCT is a life-saving intervention. However,

this causes significant disruption of the microbiome, which places these patients at very high risk for infection due to some of

our greatest antibiotic resistant threats, such as Clostridioides difficile, vancomycin-resistant Enterococci (VRE),

and multidrug-resistant Gram-negative bacteria. We are seeking to determine if coadministration of SYN-004 while the patient is

receiving IV beta-lactam antibiotics will preserve the microbiome and thus mitigate the risk from these threats," said Dr.

Erik R. Dubberke Professor of Medicine and Clinical Director, Transplant Infectious Diseases at Washington University. "There

are data to suggest preservation of the microbiome in allogeneic HCT recipients results in improved immune function after HCT as

well. This would further enhance these patients' outcomes and quality of life."

Mr. Shallcross continued, "During

the second quarter, we remained focused on executing our strategy to advance our portfolio of gastrointestinal (GI) and microbiome-focused

clinical programs. We held a highly informative pre-IND meeting with the U.S. Food and Drug Administration (FDA) for our SYN-020

intestinal alkaline phosphatase (IAP) program and clarified the parameters for IND-enabling toxicology studies. These activities

are ongoing and will support our anticipated Investigational New Drug (IND) application which we intend to file promptly during

the first quarter of 2020." Mr. Shallcross concluded, "Interest from prospective patients in our investigator-sponsored

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

strong. However, after consulting with the investigators at Cedars-Sinai Medical Center, the study sponsor, enrollment has been

extended to accommodate higher than anticipated screen-fail rates. This is, in part, due to errant laxative use during the screening

period that adversely impacts baseline measurements of IBS-C symptoms and breath-methane levels. Rigorous screening and reliable

baseline parameters are critical to all IBS-C clinical trials in order to ensure the possibility of generating a meaningful data

set of the highest quality. At this time, we are discussing with Cedars-Sinai the opportunity for a data read out in the first

half of 2020, extending our previous guidance from the fourth quarter of 2019. We look forward to sharing important updates and

progress for this and all our GI and microbiome-focused clinical programs."

Clinical Development and Operational

Quarter Ended June 30, 2019 Financial

General and administrative expenses decreased

by 27% to $1.0 million for the three months ended June 30, 2019, from $1.4 million for the three months ended June 30, 2018. This

decrease is primarily due to decreased stock-based compensation expense related to forfeitures and decreased options grants, along

with the reduction of investor relations and consulting costs. The charge related to stock-based compensation expense was $59,000

for the three months ended June 30, 2019, compared to $264,000 the three months ended June 30, 2018.

Research and development expenses decreased

by 27% to $2.6 million for the three months ended June 30, 2019, from $3.6 million for the three months ended June 30, 2018. This

decrease is primarily the result of lower SYN-004 (ribaxamase) indirect program costs for the three months ended June 30, 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, a potential Phase 1b/2a clinical trial of SYN-004 (ribaxamase) in allogeneic HCT recipients, and the continued development

of SYN-020. Research and development expenses also include a charge relating to stock-based compensation expense of $31,000 for

the three months ended June 30, 2019, compared to $293,000 for the three months ended June 30, 2018.

Other income was $80,000 for the three

months ended June 30, 2019, compared to other income of $789,000 for the three months ended June 30, 2018. Other income for the

three months ended June 30, 2019 is primarily comprised of interest income while the three months ended June 30, 2018 is comprised

of non-cash income of $783,000 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 June 30,

2019 was $21.7 million, a decrease of $7.2 million from December 31, 2018.

Synthetic Biologics will hold a conference

call today, Thursday, August 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/1096/31274. An archive of the call will be available

for replay at the same URL, https://www.webcaster4.com/Webcast/Page/1096/31274, 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 early-stage oral formulation of the enzyme intestinal alkaline phosphatase (IAP) to treat both local

GI and systemic diseases. 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 that SYN-004's unique mechanism of action designed to degrade IV beta-lactam antibiotics and prevent

dysbiosis of the gut microbiome has the potential to significantly improve outcomes for patients who undergo allogeneic hematopoietic

cell transplantation (HCT); data to suggest preservation of the microbiome in allogeneic HCT recipients results in improved immune

function after HCT as well and this further enhancing these patients' outcomes and quality of life; an IND for SYN-020 will

be filed during the first quarter of 2020; enrollment for the Phase 1b/2a clinical trial of SYN-004 is expected to begin during

the first quarter of 2020, contingent upon approval of the clinical study protocol by the Washington University School of Medicine's

Institutional Review Board (IRB) and the FDA; a data readout for the Phase 2b investigator-sponsored clinical study of SYN-010

for the treatment of IBS-C is anticipated in 1H 2020; and extending Synthetic Biologics' projected cash runway through

at least the end of Q3 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