Coulter Pharmaceutical Announces Interim Data From Phase III Investigational Trial of Bexxar; The Company Closes Phase III Enrollment.

PALO ALTO, Calif.--(BW HealthWire)--May 12, 1998--Coulter Pharmaceutical, Inc. (Nasdaq:CLTR) today announced interim data on 30 patients enrolled in the Phase III investigational trial of Bexxar(TM) (iodine I 131 tositumomab) which is being investigated for the treatment of non-Hodgkin's lymphoma (NHL).

The Phase III trial was designed to compare patients' duration of response to Bexxar with the duration of response to their prior chemotherapy. Of the 30 patients, 22 had adequate follow-up data to contribute toward the primary endpoint at time of analysis.

As assessed by the trial's clinical investigators, ten of 13 (77 percent) of the non-equivalent cases experienced a longer duration of response on Bexxar compared to only three of 13 (23 percent) patients experiencing a longer duration on the prior chemotherapy.

Nine cases resulted in an equivalent duration of response on Bexxar and prior chemotherapy. Adverse events related to Bexxar, observed to date, include a decrease in blood counts which are reversible and generally self-limiting. A minority of these heavily pre-treated patients may require support by blood transfusion or growth factor. A mild to moderate flu-like syndrome also has been observed.

The primary efficacy endpoint for this Phase III trial is the comparison of duration of response as assessed by a masked independent review panel of physicians. For the primary endpoint to be achieved, duration of response with Bexxar must be at least one month greater than duration of response with the prior chemotherapy in a significant number of patients.

The Phase III trial includes 60 NHL patients who are refractory to chemotherapy, having either no response to prior chemotherapy or disease progression within six months of their last chemotherapy regimen. The company also announced that the enrollment for this Phase III trial was closed.

"While these data are preliminary and have yet to be assessed by a masked independent panel, the data suggest that patients are tracking well to our primary endpoint by demonstrating longer duration of responses to Bexxar in the majority of the non-equivalent cases," said Michael F. Bigham, president and chief executive officer of Coulter.

"The data are particularly encouraging as these patients are refractory to chemotherapy and are expected to respond poorly to subsequent therapies at this late stage in the progression of their disease. Notably, many patients had failed aggressive `salvage' chemotherapy regimens prior to being treated with Bexxar."

Bexxar (iodine I 131 tositumomab), which is an antibody conjugated to iodine-131, attaches to a protein found only on the surface of B-cells, including both non-Hodgkin's lymphoma B-cells and normal B-cells. Coulter believes that through this targeted approach, the tumor cells receive a greater concentration of the therapeutic radiation from Bexxar relative to normal tissues.

Non-Hodgkin's lymphoma is a form of cancer that affects the blood and lymph tissues. NHL currently is the sixth leading cause of death among cancers in the United States and has the second fastest growing mortality rate.

According to statistics from the National Cancer Institute, approximately 270,000 people are afflicted with NHL in the United States alone. Of the total, the company estimates that approximately 140,000 people have low-grade or transformed low-grade disease. There are approximately 54,000 new cases of NHL diagnosed each year.

Coulter Pharmaceutical, Inc. is engaged in the development of novel drugs and therapies for the treatment of people with cancer. The company currently is developing a family of cancer therapeutics based upon two drug development programs: therapeutic antibodies and targeted oncologics.

The company's most advanced investigational biologic product candidate is Bexxar(tm) (iodine I 131 tositumomab), a monoclonal antibody conjugated to a radioisotope. The company intends to file a BLA for Bexxar by the end of 1998. Initial efforts in the targeted oncologics program are focused on the development of a tumor-activated peptide pro-drug version of doxorubicin to potentially treat certain solid tumor cancers. -0-

Except for the historical information contained herein, the matters discussed in this news release are forward-looking statements that involve risks and uncertainties, including uncertainties related to product development, uncertainties related to the need for regulatory and other government approvals, dependence on proprietary technology, uncertainty of market acceptance of Bexxar(tm) or the company's other product candidates and other risks detailed from time to time in the company's filings with the Securities and Exchange Commission (SEC). In particular, see "Risk Factors" in the company's Annual Report on Form 10-K for the year ended December 31, 1997.

CONTACT: Coulter Pharmaceutical

Sylvia Wheeler, 650/842-7306