Published in UAB Insight, Winter 2008

From Bedside to Bench and Back: A Triple Play

Masked by symptoms common to other pulmonary and cardiac diseases, pulmonary arterial hypertension (PAH) is characterized by an elevated mean pulmonary arterial pressure. Symptoms include dyspnea, chronic fatigue, syncope, and edema. Left undiagnosed or untreated, PAH leads to right ventricular failure and death. To date, it remains unclear whether PAH is truly rare, which has been the conventional thinking for years, or simply undetected and underreported (MMWR. 2005;54:SS-5).

Despite precipitating causes, PAH classification is based partially on unifying histopathologic features that produce common attributes. Plexiform lesions, medial thickening from vascular smooth muscle cell proliferation, and thrombotic lesions characterize the many forms of PAH.

Three World Health Organization (WHO) conferences have addressed the nosology of PAH, culminating in the current WHO classification system. If a distinct cause for pulmonary vascular change can be discerned, PAH is classified as associated, or APAH. The incidence of APAH linked to connective tissue disease, HIV infection, or drug exposure is difficult to measure. Approximately 10% of PAH is familial (FPAH), with an identifiable genetic mutation. Idiopathic PAH (IPAH) affects 6 people per million per year. “Aside from some aggressive malignancies, disease progression in PAH is one of the most rapid,” says cardiologist Raymond L. Benza, MD, director of UAB’s Pulmonary Vascular Disease Program. “In an untreated IPAH patient, survival is often less than 2 years,” he says.

Treatment for PAH, like that for cancer, is approached in a multitiered, multidrug fashion. Lifestyle modifications, diuretics, anticoagulants, calcium channel blockers, prostacyclins, endothelin receptor antagonists, and phosphodiesterase inhibitors are used to design complex combination therapies (Circulation. 2006:114;1417-1413).

“If our initial choice of drug therapy does not produce an optimal response, we lose precious time, and perhaps the critical window of opportunity to halt disease progression and prolong the patient’s life” Benza says. Frustrated by this hit-and-miss process, Benza, as principal investigator of a series of linked PAH projects, is exploring pharmacogenomics as an avenue to more scientifically valid initial drug choices. “What,” he asks, “if clinicians could take a blood sample and perform a rapid genetic analysis that reveals the drug or drugs that are the best initial approach for a specific individual?”

Funded by an RO1 grant from the National Heart, Lung, and Blood Institute, Benza’s pharmacogenetic/pharma-cogenomic study involves collecting samples from participants in multiple nations and ethnic groups to identify genetic components that contribute to drug response in various forms of PAH. A related study, the Pulmonary Hypertension Breakthrough Initiative, harvests lung tissue from PAH patients for IPAH research. Funded by the Cardiovascular Medical Research and Education Fund, this study will provide valuable tissue resources for molecular studies in addition to blood samples.

Pharmacogenomics

“The beauty of pharmacogenomics is that it gives us the tools to predict patient response to drug therapy,” Benza says. Results of 6-minute walk tests reflect drug efficacy and closely correlate with patient survival. If, over time, the test indicates drug-related improvements, these data propel response curves upwards.

“Response to therapy is heterogenous for PAH,” he says. “Some patients reach clinical targets, some respond partially, and some do not respond at all and die.” Unfortunately, most physicians do not take into account heterogenous responses when they read journal articles, he says. “What they see are data end points and means on a graph that reflect total, not individual, responses. We need to level the playing field, making patients the focus of new research and setting our sights on developing targeted treatments and drug therapies to produce uniform responses based on individual genetic fingerprints.”

Benza and colleagues — molecular biologist Hernan Grenett, PhD; biostatistician Christopher Coffey, PhD; research nurse manager Nancy I. Stansfield, RN, MSN; Joseph P. Barchue, PA; and William R. Prucka, a PhD candidate in UAB’s Department of Biostatistics, are the first to examine the role of pharmacogenomics and pharmacogenetics in PAH.

Benza’s team has invited 37 international sites to participate in this groundbreaking initiative. Some sites are simultaneously conducting industry-sponsored clinical trials examining the roles of bosentan and sitaxsentan therapy in PAH and have agreed to collect additional blood samples for UAB’s pharmacogenomics/pharmacogenetics project. To date, sites have sent almost 600 participant samples with matching clinical information to UAB for clinical and statistical analysis.

The pharmacogenomics/pharmacogenetics project offers multiple challenges. Stansfield manages the complexities of international study operations. “It required months of negotiations navigating language barriers and country-specific regulations to obtain the vital research potential held within these patient samples,” she says.

When samples arrive, Barchue extracts and amplifies DNA. Grenett then performs a series of complex analyses to identify the presence and nature of any gene variants or polymorphisms.

Genetic polymorphisms in enzymes, drug receptors, and drug targets have been coupled with individual variations in drug toxicity and efficacy (N Engl J Med. 2003;348:529-537) and (N Engl J Med. 2003;348:538-549). With a patient’s genotype in hand, Grenett then delves into the complex levels beneath the cell surface to investigate cellular consequences of single or multiple genomic polymorphisms.

Endothelin, a powerful vasoconstrictor, is one of several receptor pathways under investigation for a role in PAH’s pharmacogenomic pathogenesis. The cell surface receptor for bosentan, an endothelin receptor antagonist, illustrates molecular variations that can occur as a result of polymorphisms above or beneath the cell surface.

Communication systems below the cell surface catalyze multiple conformational changes that influence drug binding and regulation within the body. Polymorphisms at every step in this pathway can predict drug efficacy, and Grenett is examining each stage for genetic changes and influences.

“The endothelin system is a good example of the body’s ability to compensate for drug pathway polymorphisms,” he says. “Tailoring drugs and drug combinations to counteract the effects of polymorphisms and gain upstream pathway access when downstream pathways are blocked is one way to overcome genetic roadblocks.”

In a simple world, one genotype would determine a patient’s first-line therapy, but humans have a vast range of genetic variation. To overcome these challenges, Coffey and Prucka use statistical methods to analyze molecular data and correlate results with each patient’s clinical record. They will use complex higher order analysis to extract relationships among contributory levels of multiple polymorphic components.

“We can then predict specific drug performance against a complete backdrop of genomic contributions,” Coffey says. For example, we can determine if the drug works better or worse in patents with certain combinations of genotypes. Genotype interactions are not usually limited to one category, and “this project requires a large global sample to target four or five gene interactions,” Coffey says. “Our study will tease out complex relationships between genotypes and treatment response in patients with PAH.”

Interpreting Genetic Fingerprints

Interpretation and analysis of molecular, clinical, and genetic data will culminate in a predictive formula for individualized targeted interventions. “Our goal is to develop a clinical and pharmacogenetic formula clinicians around the world can use to predict response to therapy and determine patient survival,” Prucka says.

Benza notes elucidation of molecular drug mechanisms that affect drug targets depend on basic research. “If we can use patient samples to identify an element that drives drug response, such as a mutation that keeps a regulatory element in the ‘on’ position and limits drug efficacy, this project will move from bench-to-bedside,” says Benza, who wants to take this process a step further. “To return the focus of individualized medicine to its rightful place — the patient — we must start at bedside, go to the bench to find a solution, and then take the answers back to the bedside — a triple play,” he says.

For more information:
Dr. Raymond Benza
Dr. Hernan Grenett
1.800.UAB.MIST
mist@uabmc.edu