SR. Director of Business Development

EXHIBIT 99.1 Company Contact: Media Contacts: Andrew Wiseman, Ph.D.
Sr. Director of Business Development
and Investor Relations
La Jolla Pharmaceutical Company
858-646-6615
andrew.wiseman@ljpc.com Virginia Amann (608-274-6046)
Carin Canale (858-336-3027)
Atkins + Associates
for La Jolla Pharmaceutical Company
vamann@irpr.com

LA JOLLA PHARMACEUTICAL ANNOUNCES RESULTS OF
PHASE III TRIAL OF RIQUENT TM

SAN DIEGO, February 18, 2003 La Jolla Pharmaceutical Company (Nasdaq: LJPC) today announced preliminary findings from a Phase III clinical trial evaluating RiquentTM, previously known as LJP 394, for the treatment of lupus renal disease. The Company continues to analyze the results from the trial.

The objective of the study was to determine Riquent's ability to delay the following: renal flare, treatment with HDCC (high-dose corticosteroids and/or cyclophosphamide) or other immunosuppressive drugs, hospitalization, or death due to lupus (Major SLE flare); reduce antibodies to double-stranded DNA (dsDNA); and to assess safety compared with placebo in the intent-to-treat population and in patients with impaired renal function at baseline. The intent-to-treat population was defined as patients with high-affinity antibodies to Riquent. The trial was designed to compare the effect of drug or placebo treatment in two well-balanced groups of lupus patients with a history of renal disease. Additional information follows in the section titled Trial Design.

Summary of Results

Intent-to-treat analysis: Riquent appeared to be well tolerated with no apparent differences in the overall incidence of serious adverse events or adverse events between Riquent-treated and placebo-treated patients. However, an initial assessment of the trial data indicates that treatment with Riquent did not increase length of time to renal flare, the primary endpoint, in a statistically significant manner when compared with placebo through the end of the study. There were 298 patients in the intent-to-treat population, 145 on Riquent and 153 on placebo. Patients were treated for as long as 92 weeks with a median of 46 weeks.

There was a statistically significant reduction in antibodies to dsDNA in the Riquent-treated group compared with the placebo-treated group (p<0.001). Antibodies to dsDNA are believed to result in renal flares and other clinical manifestations of lupus. Riquent was designed to reduce antibodies to dsDNA and this effect has been demonstrated in all clinical studies of Riquent.

In this study, changes in antibodies to dsDNA strongly correlated with changes in complement levels in an inverse relationship (p < 0.001). Complement C3 levels below


normal at baseline (hypocomplementemia) strongly correlated with an increased risk of renal flare (p< 0.001). Similar correlations between antibody levels and complement C3 were observed in the previous Phase II/III study. Together, these data confirm the pathogenic nature of these antibodies to dsDNA in lupus patients.

In the intent-to-treat population, there were fewer renal flares, treatments with HDCC and Major SLE (systemic lupus erythematosus) flares in Riquent-treated patients compared with placebo-treated patients. The estimated median time to renal flare was 123 months in the Riquent-treated group and 89 months in the placebo-treated group. There were 41 renal flares, 17 (12%) in Riquent-treated patients and 24 (16%) in placebo-treated patients. There were 68 treatments with HDCC, 32 (22%) in the Riquent-treated group and 36 (24%) in the placebo-treated group. There were 82 Major SLE flares in the trial, 35 (24%) in patients on Riquent and 47 (31%) in patients on placebo. None of these differences were statistically significant.

Reviewing a graph of the results showed that the Riquent and placebo lines for time to renal flare and for the changes in antibody levels were separating until weeks 46 to 48. In the first 46 weeks, 90% or 22 of 24 renal flares occurred in the study in the placebo patients compared with 59% or 10 of 17 in the Riquent-treated patients. At weeks 44, 46, and 48, the incidence of renal flares was 20:10 (p = 0.085), 22:10 (p = 0.041) and 22:11 (p = 0.067), respectively, in favor of Riquent. At weeks 44, 46 and 48, the incidence of renal and/or Major SLE flares was 43:27 (p=0.057), 46:28 (p=0.033) and 46:29 (p=0.061), respectively, in favor of Riquent. More discussion follows in the section titled Other Observations.

In addition, the results from this trial appear to support the use of the Company's high-affinity assay to identify patients who may respond to Riquent and also to confirm the high-affinity analysis approach used in the previous Phase II/III study. As the baseline affinity of patients' antibodies for Riquent increased, the number of renal flares declined in the Riquent-treated group compared with the placebo-treated group.

Patients with Impaired Renal Function analysis : In a prospectively defined subpopulation with impaired renal function at baseline, defined as a serum creatinine b31.5 mg/dl at baseline, there were 43 patients, 20 on Riquent and 23 on placebo. Riquent-treated patients had fewer renal flares, treatments with HDCC and Major SLE flares compared with patients on placebo, but the number of patients was small and the differences were not statistically significant. There were 8 renal flares, 2 (10%) in patients on Riquent and 6 (26%) in patients on placebo. There were 9 treatments with HDCC, 3 (15%) in patients on Riquent and 6 (26%) in patients on placebo. There were 11 Major SLE flares, 4 (20%) in patients on Riquent and 7 (30%) in patients on placebo. There were 14 renal flares and/or Major SLE flares, 5 (25%) in patients on Riquent and 9 (39%) in patients on placebo. Similar results in the same group were observed for renal flares in a previous Phase II/III study: no renal flares (0%) were observed in the 11 Riquent-treated high-affinity patients compared with 6/11 (55%) of the placebo ?treated high-affinity patients. Experts believe a delay in time to or a decrease in the incidence


of renal flares and/or Major SLE flares in this high-risk population would be con ...