Researchers from Biogen Idec Inc and Elan Corporation Inc say they have developed an enzyme-linked immunosorbent assay (ELISA) that can detect JC virus (JCV) antibodies

Megan Brooks – Authors and Disclosures

They hope that with further research the assay could be used to identify patients with multiple sclerosis (MS) at greatest risk of developing progressive multifocal leukoencephalopathy (PML) associated with natalizumab treatment (Tysabri) and help guide use of this immunosuppressive agent.

Data from preliminary analyses of the assay were released online August 24 in Annals of Neurology and will appear in the journal’s September issue.

PML is a rare but serious and often fatal adverse effect of natalizumab, characterized by progressive damage or inflammation of the white matter of the brain at multiple locations. Infection with JCV is a prerequisite for PML development.

Natalizumab was temporarily pulled from the market in 2005 after being linked to PML but was reintroduced in 2006 with stricter safety warnings. To date, there are 63 confirmed cases of natalizumab-associated PML, including 12 deaths, according to a statement issued Tuesday by Elan Corp. “The global PML rate is still within the 1 in 1,000 rate previously seen in clinical trials,” the statement reads. Some 52,700 patients continue to take the drug around the world, according to the company.

In the new report, Leonid Gorelik, PhD, from Biogen Idec Inc, and colleagues developed and analytically validated a “sensitive and specific” 2-step ELISA for anti-JCV antibodies. (Development and testing of the assay was funded jointly by Biogen Idec Inc and Elan Corporation Inc.)

Using ELISA in a large, geographically diverse sample of 831 MS natalizumab-treated MS patients, the researchers found that 53.6% tested positive for anti-JCV antibodies (95% confidence interval, 49.9% – 57.3%). “The false-negative rate of the ELISA was calculated to be approximately 2.5%, with an upper 1-sided confidence limit of 4.4%,” they report.

This assay “suggests that only 53% of the population has been exposed to the JC virus as opposed to earlier studies, which suggest that close to 80% of the population had been exposed,” Lily Jung Henson, MD, director of the neurology clinic at the Swedish Neuroscience Institute/Swedish Physicians Division, Seattle, Washington, and member of the American Academy of Neurology, noted in an email to Medscape Medical News.

“This is obviously good news in terms of the risks of developing PML being less in the general population than originally thought,” said Dr. Jung Henson, who was not involved in the research.

“Notably,” Dr. Gorelik and colleagues say, all 17 (100%) of the pre-PML samples available from natalizumab-treated patients who were eventually diagnosed as having PML were seropositive for anti-JCV antibodies.

“If these numbers can be confirmed,” Dr. Jung Henson said, “it means we can identify which patients have not been previously exposed to the JC virus and therefore can be treated with natalizumab without fear of developing PML. Those who are positive will make a more informed decision about the risks they face in taking natalizumab.”

Dr. Gorelik and colleagues say further clinical studies are under way to confirm their results “and to determine whether our assay may be a useful tool for stratifying natalizumab-treated MS patients for higher and lower risk of PML.”

In a companion paper in Annals of Neurology, Richard A. Rudick, MD, from the Cleveland Clinic Mellen Center for Multiple Sclerosis in Ohio, and colleagues from Biogen Idec and elsewhere report that measuring JCV DNA in blood or urine using currently available methods is “unlikely to be useful at predicting PML risk in natalizumab-treated MS patients.”

Their findings are based on a total of 12,850 blood and urine samples from nearly 1400 MS patients from natalizumab clinical trials.

Using the ViraCor quantitative polymerase chain reaction (PCR) assay, they detected JCV DNA in 4 of 1397 patients (0.3%) from the Dose Suspension Safety Assessment study. A more sensitive quantitative PCR from the National Institutes of Health confirmed these positive samples and detected JCV DNA in an additional 2 of 205 patients (1%) who tested negative with the ViraCor assay, they say.

None of these 6 JCV DNA–positive, natalizumab-treated patients developed PML, the investigators report.

In the Safety of Tysabri Redosing and Treatment, or STRATA, study, JCV DNA was detected in plasma of 6 of 1094 patients (0.3%), none of whom developed PML. Urine was assessed at baseline and week 48 of natalizumab treatment in 224 of these patients, yielding positive results in 58 (26%) at baseline and 55 (25%) at week 48. JCV DNA was not found in any peripheral blood mononuclear cells (PBMCs) from any of the 1094 patients before or after natalizumab treatment.

“In 5 patients who developed PML,” the researchers report, “JCV DNA was not detected in blood at any time point before symptoms first occurred.

“The commercial test used to test the presence of JC virus DNA in blood and urine was not useful in identifying patients who developed PML and showed presence of JC virus in those who did not go on to develop PML,” Dr. Jung Henson noted.

Dr. Rudick and colleagues point out that the rates of JCV DNA in blood and urine of natalizumab-treated patients in their analyses are consistent with several other reports from smaller numbers of patients that found increased JCV DNA in plasma, PBMCs, or urine after 15 to 30 months of treatment.

They are inconsistent, however, with 2 recent reports, in which JCV DNA was detected in urine from 64% of 19 patients treated with natalizumab for 1 year and in plasma and PBMCs after treatment for 18 months (N Engl J Med. 2009;361:1067-1074 and J Neurol.2010;257:264-272).

Possible explanations for the discrepant findings include “differences in assay methodologies, sample handling (particularly in samples with low DNA copy number), and chance occurrence in a small cohort,” Dr. Rudick and colleagues speculate.

The first study was jointly funded by Biogen Idec Inc and Elan Corporation. All of the study authors of the first paper are employees of these companies. The data for the second report were derived from the National Institutes of Health Laboratory of Molecular Medicine and Neuroscience and were shared with Biogen Idec under a collaborative agreement. Dr. Rudick and 3 coauthors on the second study have received consulting fees from Biogen Idec. The other coauthors are employees of Biogen Idec. Dr. Jung-Henson has disclosed no relevant financial relationships.

Ann Neurol. Published online August 24, 2010.

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