Associate Professor, Department of Anatomy and Cell Biology, ETSU “CAN GENETIC VARIATION BE USED TO PREDICT RESPONSE TO PSYCHOTROPIC DRUGS?” BARNEY E. MILLER, Ph.D. Associate Professor and Director of Research, Department of Psychiatry and Behavioral Sciences Associate Professor, Department of Anatomy and Cell Biology, ETSU

yes But not as well as this fellow.

……more about this later. Only product on the market (FDA approved 2006) in the US in the field of pharmacogenetics: Roche’s AmpliChip® CYP450 Test. Identifies patient metabolic phenotype based on their genetic variability in two cytochrome P450 (CYP450) enzymes. Test rates patient as one of 4 classes in terms of speed of drug metabolism. Used as a guide for dosing, including psychotropic drugs. Uses PCR and Affymetrix high-density microarray technology. ……more about this later.

Individuals have different responses to treatment Obviously not every member of a species responds in the same way to a given dose of a drug. Some differences are subtle, eg. Person A undergoes remarkable improvement while Person B improves but less well than desired, Person C - no improvement. WHY?

Why do individuals have different responses to treatment? Genetic differences Linkage has been made between “specific genetic variations and treatment response” in a few drugs, eg.; P450 2D6 gene (metabolizes many psychotropic drugs) This linkage provides some guidance to the clinician in drug choice In the above case the drug choice is guided by the “type of metabolizer” identified by polymorphisms in the 2D6 gene Environmental or circumstantial differences

SNP *SNP = pronounced “snip”, a single nucleotide polymorphism, a single chemical group (nucleotide) difference in the DNA code of a gene. Although all members of a biological species “share” the same fundamental DNA there are many differences within the coding regions for genes. Some of these differences involve significant variations in phenotype; some involve smaller much less obvious changes. Some of these differences produce no detectable phenotypic variation, some produce subtle variations (enzyme activity, receptor binding) and some produce obvious differences (hair color, height, etc.) The SNP’s that we’re interested in are those changes that lead to subtle variations in phenotypic response to medicines.

Some examples:

Where does one start looking? If genetic variation via SNPs can be used to guide the pharmacological treatment of depression then…. Where does one start looking? How might such small changes in a gene be involved? Metabolically? Via “drug site of action proteins”? Via control processes; eg., transport and activation systems associated with “site of action proteins”?

Depression In 2001 there were 24.5 million visits (~8% US pop.) to physicians for depression treatment(1). 89% of these cases received medication The top 5 medications were: sertraline (18%), paroxetine (16%), fluoxetine (14%), citalopram (13%), and bupropion (9%) The lifetime prevalence for depression in the US is estimated at ~16%(2) Stafford RS et al. Primary Care Companion J Clin Psychiatry 2001; 3(6) p232-235 (2) Nestler EJ et al. Neuron 2002,34 p13-25.

Treatment Most physician administered cases of depression are treated with antidepressant drugs of the following two categories: SSRI; eg., Prozac, Zoloft, Celexa, Paxil Non-SSRI (SNRI, SDRI); eg., Wellbutrin, Effexor, or Duloxetine

Standard of care*: A patient is started at a low to mid-range dosage usually with an SSRI Clinical improvement is usually expected at 2-3 weeks If no clinical improvement in 4-6 weeks: Dosage may be increased Drug may be switched Another drug added Non-medical augmentation The time to clinical improvement (discovery of an efficacious drug/combination) varies from 1-12 months. *Mann, JJ. NEJM 2005; 353:1819-1834.

Current Pathway to Finding the Right Drug Therapy for the Patient 2nd Medication Adjustment / Change 1st Medication Adjustment / Change 4th Medication Adjustment / Change Initial Patient Visit w/ Dr. I think the doctor has finally found the right medication at the right dose for me. 3rd Medication Adjustment / Change Copyright 2006, The Fallon Group, LLC

Primary Challenges for the Physician are… Which therapeutic class will the patient best respond to? Which drug within the class? So which patient gets which class? ? ? ? SSRI’s SNRI’s Other ? ? Paxil® Zoloft® Effexor® Prozac® Wellbutrin® Cymbalta® Luvox® Copyright 2006, The Fallon Group, LLC

Individual Response to Treatment 30 – 50% of patients have substantial residual symptoms after adequate first-line treatment *. The choice of starting drug is largely based on the experience of the clinician with little rational basis. All antidepressants do work – they reduce or eliminate the symptoms of depression in some, but not all people. No genetic differences are currently used to guide drug choice. *Mann, JJ. NEJM 2005; 353:1819-1834.

Pharmacogenetics of Antidepressant Drug Response Variation in clinical response is a critical problem in the management of depression*. Polymorphisms in genes encoding neurotransmitter receptors, particularly serotonin, are prime targets for pharmacogenetic research. STAR*D, a large multi-center study, is currently evaluating some 768 SNPs associated with 68 genes**. *Malhotra AK et al. Am J Psychiatry 2004; 161: 780-796. **McMahon FJ et al. Am J Human Genetics 2006; 78:804-814.

The C(-1019)G SNP In the promoter region of the serotonin receptor 1A a polymorphism occurs as a C (cytosine) to G (guanine) difference. Thus someone will be CC, CG or GG at this site. The GG occurs in 28% of the depressed group and 11% in the controls(1) Thus in the US of the 24.5 million depression cases/year (2001) 6.8 million might be expected to be the GG type. (1) Lemonde S, et al. J Neuroscience, 2003, 23(25) p. 8788-8799.

Serotonin 2A SNP In addition to the C(-1029)G polymorphism in the 1A receptor, we also sequenced a SNP in the 2A receptor T(102)C reported to be associated with SSRI intolerance *. *Murphy GM, et al. Am J Psychiatry 2003; 160:1830-1835.

How’s it work? The serotonin 1A receptor is a sort of flow control receptor. It is found on the dendritic field of a neuron (near the cell body) and it causes the neuron to slow its firing rate when it is activated by the presence of serotonin. It is a negative feedback control on the neuron. The GG people produce more of this receptor because of reduced binding for a repressor factor at the 1A’s gene promoter region.

Too much 1A receptor? If a person makes too much 1A receptor their serotonergic neurons are rendered less active, excitable or capable of normal responsiveness in the presence of serotonin. This condition directly opposes the action of SSRI drugs which work to keep more serotonin available to neurons.

Our finding In an on-going observational study of depressed patients, we have correlated genotype at the C(-1019)G with patient history and measures of depression. Blood was obtained by finger prick and cycle sequenced in both directions using primers designed for this purpose. Results: A disproportional number of individuals homozygous for G(-1019) were found to be currently prescribed a drug that was not a selective serotonin reuptake inhibitor (SSRI; p<0.025, n=48). Preliminary data suggest that these individuals showed significantly less improvement with drug therapy based on two independent measures (p = 0.0008, PHQ9; p =0.02, CGI). The improvement of only G/G patients tended to correlate with the NAT/SERT selectivity of the drug (p = 0.075, n =14) and with the DAT/SERT selectivity of the drug (p=0.08, n=14). SERT=serotonin transporter, NAT=norepinephrine transporter, & DAT=dopamine transporter

Correlation to Drug Potency Drug potency (expressed as inhibition constants in nmoles) ratios were calculated for each GG subject’s current medication Lower number = greater potency

Selectivity = (ki NAT / ki SERT)

Selectivity = (ki DAT / ki SERT)

Selectivity = (ki NAT / ki SERT)

Our Findings on the Serotonin 2A Receptor SNP A previous study compared intolerance to paroxetine (an SSRI) compared to a non-SSRI as a function of genetic markers. Paroxetine side effect severity was associated with the C/C genotype of the 2A T(102)C SNP *. Since only one SSRI was included, the specificity of this association with paroxetine could not be addressed. Our data suggest that the drug intolerance of the C/C genotype is selective for paroxetine. Due to limited number of subjects this finding needs additional study. *Murphy GM, et al. Am J Psychiatry 2003; 160:1830-1835.

Failure rate = stopped drug due to side effects, based on Subject history

Utility Determination of a patient’s genotype at this serotonin receptor loci may allow the prescribing physician to make a rational decision with respect to initial antidepressant drug choice. These patients are relatively resistant to current pharmacotherapy especially to the SSRI class Should be treated more aggressively and with NE or DA class antidepressants (eg., Buproprion, Venlafaxine, Duloxetine) Most of the GG patients in our study have been on more than 3 SSRI’s (at various doses and combinations) prior to starting Non-SSRI type drugs. This means that their medications were being adjusted for up to a year.

Need for more subjects We need to recruit at least 30 more subjects for this study. Anyone who has a primary diagnosis of depression and is taking any standard antidepressant qualifies. Contact Chris Newell (439-2126)or myself (439-2121) for more information.