Cubist to Feature Antibiotic Development Programs at 52nd Annual Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC)

Cubist Pharmaceuticals, Inc.Eileen C. McIntyre, 781-860-8533Senior Director, Investor RelationsorCubist Pharmaceuticals, Inc.Francis McLoughlin, 781-860-8777Director, Corporate Communications

(NASDAQ: CBST) today provided an overview of selected studies that will be presented at the 52 Annual Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) taking place from September 9 to 12 in San Francisco.

The studies focus on the company’s lead investigational antibiotics, CXA-201 and CB-315, basic science research as well as the marketed antibiotic, CUBICIN® (daptomycin for injection).

“The breadth of presentations at ICAAC shows how we are advancing science to not only inform the appropriate use of CUBICIN but also to discover and develop new antibiotics for serious and potentially life-threatening infections,” said Steve Gilman, PhD, Chief Scientific Officer at Cubist Pharmaceuticals. “We’re excited that our lead pipeline antibiotics are now in pivotal Phase 3 trials and we hope to one day make them available to doctors and patients who deal with the challenge of emerging resistant infections.”

A list of selected presentations can be found on the company’s . Key highlights include:

CXA-201, a novel cephalosporin in combination with tazobactam, is in development for the treatment of certain serious infections caused by multi-drug resistant (MDR) Gram-negative organisms, particularly and those in the Enterobacteriaceae family.

This year at ICAAC, six studies on CXA-201 will be presented. In one important study, CXA-201 showed greater potency than currently available anti- cephalosporins (ceftazidime and cefepime) and piperacillin/tazobactam when tested against and Enterobacteriaceae strains from hospitals in the United States.

(Abstract E-199) Sunday, September 9, 2012, 11:30 a.m. - 1:30 p.m. PDT, Halls A-C.

CB-315, a novel oral antibiotic, is in development for the treatment for -associated diarrhea, or CDAD. causes diarrhea linked to 14,000 American deaths each year. This year the company initiated pivotal Phase 3 studies of CB-315, and data at ICAAC will show that is more susceptible to CB-315 than to other antibiotics like vancomycin and moxifloxacin.

Clostridium difficile (Abstract E-807) Monday, September 10, 2012, 11:15 a.m. - 1:15 p.m. PDT, Halls A-C.

Several abstracts on basic science and drug discovery efforts will be presented at the meeting. One of these focuses on CB-027, which is a novel broad spectrum cephalosporin with potent activities against both Gram-positive and Gram-negative bacteria. This includes methicillin-resistant (MRSA) and . In this animal study, CB-027 showed potent activity against several drug-resistant strains, including MRSA, ceftazidime-resistant and .

In vivoStaphylococcus aureusPseudomonas aeruginosaKlebsiella pneumoniae (Abstract F-846) Monday, September 10, 2012, 11:15 a.m. - 1:15 p.m. PDT, Halls A-C.

More than 30 scientific abstracts at ICAAC feature data on daptomycin. Importantly in an oral presentation, new data will show that early use of daptomycin significantly lowers rates of clinical failure, including both 90-day mortality and microbiological failure compared to vancomycin for infections caused by certain strains (VAN MIC > 1 mg/L) of MRSA.

Staphylococcus aureus (Abstract K-1121) Monday, September 10, 2012, 2:45 a.m. - 3:00 p.m. PDT, Room 132.

CUBICIN® (daptomycin for injection) is approved in the U.S. and many other non-US markets as therapy for bloodstream infections (bacteremia), including right-sided endocarditis, caused by methicillin-resistant (MRSA) and methicillin-susceptible (MSSA), and complicated skin infections caused by certain Gram-positive bacteria, including MRSA. CUBICIN is not indicated for the treatment of pneumonia. Most adverse events reported in clinical trials were mild to moderate in intensity. The most common were anemia, constipation, diarrhea, nausea, vomiting, injection site reactions, and headache. To reduce the development of drug-resistant bacteria and maintain the effectiveness of CUBICIN, CUBICIN should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria susceptible to CUBICIN. For full prescribing information, including important safety information, please visit .

Bacteria are categorized in two main classes, Gram-positive and Gram-negative. This classification is based on the structure of the bacterial cell and has implications as it relates to antibiotics. Both Gram-positive and Gram-negative bacteria cause infections. Resistance to commonly-used antibiotics is becoming increasingly prevalent in both Gram-positive and Gram-negative bacteria. Gram-positive bacteria, such as generally have a thick outer cell wall and a single cell membrane, whereas Gram-negative bacteria, like , have an outer cell membrane and a thinner cell wall. Infection-causing Gram-positive bacteria include and Diseases caused by these Gram-positive bacteria include infections caused by Methicillin-Resistant commonly referred to as MRSA, and CDAD. Examples of infective Gram-negative bacteria include ,, , and . The diseases caused by Gram-negative bacteria include peritonitis (a type of abdominal infection), septicemia (blood infection), pneumonia, neonatal (newborn) meningitis, urinary tract infections, intra-abdominal infections and burn and wound infections.

is one of the most prevalent Gram-negative bacteria responsible for hospital-acquired infections (also known as nosocomial infections). Nosocomial infections are becoming increasingly common in intensive care units (ICU) and data from the National Nosocomial Infections Surveillance of ICUs in the United States shows that is the most frequently isolated Gram-negative cause. Pseudomonal infections in the hospital causing pneumonia and urinary tract infections have almost doubled between 1975 and 2003. Similar increases in Pseudomonal infections were observed in Europe SENTRY Antimicrobial Surveillance Program, between 1997 and 2002. Pseudomonal infections can involve any part of the human body, but among the most common are lung urinary tract, bloodstream, wound/burn, and intra-abdominal infections. Resistance to current treatment regimens for such infections is growing, with the increasing appearance of strains expressing multi-drug resistance against commonly used anti-pseudomonal antibiotics.

CDAD is a disease caused by an overgrowth of, and toxin production by , a Gram-positive bacterium naturally found in the lower gastrointestinal tract. This overgrowth is caused by the use of antibiotics for the treatment of common community and hospital acquired infections. Many antibiotics cure the underlying infection but, as a consequence, disrupt the natural balance of intestinal bacteria which to overgrow. The overgrown bacteria produce enterotoxin and cytotoxin, two proteins that can lead to potentially life-threatening severe diarrhea and sepsis (blood infection). CDAD rates and severity are increasing, due in part to the spread of a new strain with increased virulence and greater resistance to fluoroquinolones, a standard of care treatment. According to an article in the October 2008 issue of the , during the mid- and late-1990s, the reported incidence of infections caused by in acute care hospitals in the United States remained stable at 30 to 40 cases per 100,000. However in 2001, this number rose to almost 50, with subsequent increases to the point that the number of cases that were reported in 2005 (84 per 100,000) was nearly three times the 1996 rate (31 per 100,000).

Cubist Pharmaceuticals, Inc. is a biopharmaceutical company focused on the research, development, and commercialization of pharmaceutical products that address significant unmet medical needs in the acute care environment. Cubist is headquartered in Lexington, Mass. Additional information can be found at Cubist’s web site at .

Cubist and CUBICIN are registered trademarks of Cubist Pharmaceuticals, Inc.