Microarrays provide clues to drug side effects

The US Food and Drug Administration and Applied Biosystems researchers have recently collaborated on a comparative study of the genetic effects of the three commercialised glitazone drugs, using the company's microarray platform technology.

Gene-expression microarrays are one of the most promising technologies for detecting potential adverse side effects.

Modern drugs can cost US$800 million or more to bring to market, and -- as recent scandals involving high-profile drugs have shown -- anticipating side effects is becoming increasingly important.

In 2000 reports of severe liver toxicity caused the FDA ordered Parke-Davis/Warner-Lambert to yank its type 2 diabetes drug Rezulin (troglitazone) off the market.

Rezulin's mechanism of toxicity was unknown, but the episode cast a pall over the future of an entire class of anti-diabetic drugs called giitazones, including GlaxoSmithKline's Avandia (rosiglitazone), and Takeda's Actos (pioglitazone).

This year Merck was forced to pull its COX-2 inhibitor Vioxx (rofecoxib) off the market. Initially hailed as a wonder drug for arthritis, Vioxx now stands accused of causing premature heart attack and stroke in up to 100,000 patients.

Development costs aside, the damages bill for Merck alone could run to US$10 billion plus. Pfizer's Celebrex remains on the market, but all COX-2 inhibitors are now running under a yellow flag.

A new blockbuster drug can become a bank-breaker over night if long-term use by several million patients reveals unanticipated toxic side-effects that escaped detection in clinical trials - principally because there was no theoretical or experimental reason to test for them.

Japanese pharma Takeda developed the first drug in the class, ciglitazone, as a lipid-lowering agent, but halted development when it exhibited liver toxicity. It subsequently developed and marketed a modified molecule, pioglitazone (Actos) as an anti-diabetic.

The Rezulin episode rang alarm bells in the FDA, which sought expressions of interest in developing new technologies to evaluate drugs for potential toxicity before they can reach the market - technology that might also be embraced by big pharma companies anxious to minimise their commercial risks and exposure to the cost of damages from class-action suits.

Genomics clues

Dr Lu Zhang, of Applied Biosystems' Arrays Product Development Group in Foster City, California, described the project's findings to a systems biology workshop at Melbourne's La Trobe University.

"We began marketing our microarray system two years ago, and we are actively looking for collaborators, and biological validation of the platform," Zhang said.

"The FDA published voluntary guidelines for submitting genomics data for new drugs in March this year. They want to get more knowledge before they implement it in their risk-evaluation processes."

The FDA-Applied Biosystems study used AB's Expression Array System to compare drug-induced expression changes across the genome in rat primary hepatocytes exposed to the three commercial glitazone drugs - Rezulin, Avandia, Actos, and the Takeda's prototypic ciglitazone.

The anti-diabetic glitazones all block an hepatocyte receptor, PPARgamma (peroxisome proliferator activated receptor gamma).

The rat hepatocytes were treated with two 30-nanomolar doses of each drug at 6 and 10 hours. The microarray system highlighted 250 genes that were differentially expressed between the four different treatments.

Zhang said Rezulin exhibited the same effects on gene expression as Takeda's discarded ciglitazone, while Takeda's Actos and GSK's Avandia, which group structurally, exhibited minimal toxicity - in accord with both clinical and epidemiologal data.

The microarray system showed that Rezulin and ciglitazone both elevated genes that map to several important biochemical pathways in liver cells - the MAPK (MAP kinase), LDL (low-density lipoprotein) and TNF (the inflammation inducer Tumor Necrosis Factor).

In contrast, Avandia and Actos selectively elevated the expression of genes in the PPAR-activation pathway, with minimal effects on the non-target MAPK, LDL and TNF pathways. The two drugs get a qualified tick - they are also known to increase the susceptibility of Type 2 diabetics to obesity, a prime risk factor for cardiovascular disease. People taking Avandia and Actos have to go on special diets.

Zhang said the findings provided clues that could help elucidate the mechanism of hepatotoxicity induced by Rezulin.

She said that ideally, microarray technology could help flat indications of toxicity very early in the drug-development cycle, so that companies could avoid the enormous costs involved in the approval process for new drugs - and the costs involved in litigation if drugs are found to be toxic in general use.