JOINT DEVELOPMENT AND COLLABORATION AGREEMENT

Par Pharmaceutical Exhibit 99.1

Contacts:

Stephen J. Mock

Cecelia C. Heer

Par Pharmaceutical Companies, Inc.

(201) 802-4000

PAR PHARMACEUTICAL ANNOUNCES EQUITY INVESTMENT

IN OPTIMER PHARMACEUTICALS

PAR ENTERS INTO JOINT DEVELOPMENT AND COLLABORATION AGREEMENT WITH OPTIMER

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COMPANIES TO COLLABORATE IN THE DEVELOPMENT AND MARKETING

OF ANTIBIOTIC, PAR-101;

PAR-101 REPRESENTS PROMISING ADDITION TO PAR'S EXPANDING PIPELINE OF

NEW DRUG CANDIDATES

SPRING VALLEY, NEW YORK, MAY 3, 2005 - Par Pharmaceutical Companies, Inc.

(NYSE:PRX) today announced that it has entered into a purchase agreement to

acquire a 16 percent equity interest in Optimer Pharmaceuticals, Inc. Optimer is

a privately held biotechnology company located in San Diego, California. The

company was founded in 1998 by Michael Chang, Ph.D., Professor Chi-Huey Wong of

Scripps Research Institute and Professor Samuel Danishefsky of Memorial

Sloan-Kettering Cancer Center.

Par and Optimer have also entered into a joint development and collaboration

agreement for the compound now known as PAR-101. PAR-101 is a narrow-spectrum

antibiotic with potent activity against CLOSTRIDIUM DIFFICILE (C. DIFFICILE). It

is orally active, with minimum systemic exposure, and is being developed for the

treatment of C. DIFFICILE-associated diarrhea (CDAD), the leading cause of

institutional enteric infections. Currently, there is no satisfactory treatment

for CDAD. PAR-101 has been granted Fast-Track status by the U.S. Food and Drug

Administration, and is now in Phase IIa clinical studies, with Phase IIb trials

expected to commence in the second half of 2005. If all proceeds well with the

product's development and regulatory review, Par believes that PAR-101 could be

available commercially as soon as 2008.

"Working closely with our accomplished colleagues at Optimer, we look forward to

demonstrating the utility of PAR-101 in addressing a significant, unmet medical

need," said Shankar Hariharan, Ph.D., executive vice president and chief

scientific officer of Par. "We are excited to add this new compound to Par's

expanding roster of new drug candidates in clinical development.

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"CDAD imposes significant costs on society both in terms of human suffering and

economics. We believe that PAR-101 has the potential to become an important

addition to the physician's armamentarium for treating this debilitating

infection," Dr. Hariharan added.

"We are proud and excited to have Par as our development and marketing partner,"

said Dr. Michael Chang, president and chief executive officer of Optimer. "We

believe that this collaboration will significantly expedite the

commercialization of PAR-101."

In the U.S., as many as 10 percent of all patie