An HIV/AIDS treatment developed by GlaxoSmithKline (NYSE:GSK) is now being studied with Siemens (NYSE:SI) to develop companion diagnostics that doctors can use to select patients appropriate for the treatment.

The partnership with ViiV Healthcare, GSK’s AIDS drug joint venture with Pfizer (NYSE:PFE), will study the drug Selzentry in a phase 3 study comparing it to another drug. The partners also plan to develop tests to help doctors make treatment decisions about the drug. Selzentry defends T-cells against the R5 form of HIV. It works by keeping R5 from entering T-cells. The drug generated $177 million in 2011 sales for ViiV, which bases its U.S. operations in Research Triangle Park, North Carolina.

Siemens is also partnering with San Diego, California company Tocagen to start tests supporting clinical trials on a Tocagen investigational gene therapy for treating primary brain cancer. The partners also plan to develop companion diagnostics to monitor the therapy.

Tests for the ViiV and Tocagen treatments would need U.S. Food and Drug Administration approval, but they are part of the expansion of personalized medicine efforts into additional therapeutic areas. Such companion diagnostics are paired with a particular drug or therapy and their use helps doctors make treatment decisions, such as determining whether a patient will respond to a particular therapy or assessing the likelihood of side effects.

Market research firm Visiongain values the annual companion diagnostics market at more than $1.5 billion worldwide. The firm says that although the market is small right now, it is one of the fastest growing segments of the in vitro diagnostics market. Beyond the development of companion diagnostics for physicians to use on patients, Visiongain notes that tests can be used for drug discovery and development.

Financial terms for the ViiV and Tocagen partnerships with Siemens were not disclosed. But Siemens’ companion diagnostics unit has taken steps to establish more relationships with pharma companies to help with drug development in clinical trials and also to develop and commercialize new companion diagnostics for use with patients.

ViiV has started phase 3 trials comparing Selzentry with Gilead Sciences (NASDAQ:GILD) drug Truvada, both in combination with another AIDS drug, darunavir/ritonavir. The trial is the first large phase 3 study that will compare a genotypic test with a phenotypic test to identify patients who are appropriate for treatment with Selzentry.

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