The Role of Neuroimaging in Clinical Trials and Drug Discovery In Psychiatry

1. The Role of Neuroimaging in Clinical Trials and Drug Discovery In Psychiatry Joseph C. Masdeu, MD, PhD Section on Integrative Neuroimaging National Institutes of Health

2. Neuroimaging in Drug Discovery and Development • Preclinical models • Early clinical experiments to demonstrate 'proof of biology' • Linking target engagement (TE) to • drug-induced biological changes expected to give clinical benefit • Clinical trials to demonstrate proof of concept (PoC), • Engaging a particular target is linked to • a meaningful change in a clinical end point • thus demonstrating a new avenue to treat a condition in patients

3. The Glycine Transporter 1 and Schizophrenia In schizophrenia NMDA function is impaired Lisman JE et al. Trends Neurosci. 2008;31:234 GlyT-1 regulates glycine levels at NMDA sites Glycine is an essential co-agonist of NMDA receptors GlyT-1 inhibitors enhance NMDA function (preclinical studies) Selective GlyT-1 inhibitors are being studied as drug candidates for schizophrenia Catafau A et al. Neuroreceptor Meeting. Pittsburgh 2008

4. [11C]GSK931145: A New PET Ligand for Glycine Transporter 1 • Transporter characteristics • Autoradiography • Physiology • Pharmacology

5. [11C]GSK931145: A New PET Ligand for Glycine Transporter 1 Pig Primate Human [11C]GSK931145 distribution in a healthy volunteer

6. [11C]GSK931145: A New PET Ligand for Glycine Transporter 1   • This new GlyT-1 PET ligand is being applied to: • Understand the role of GlyT-1 in Neuropsychiatric disorders • Drug development • pharmacokinetics (PK) • receptor occupancy (RO) • pharmacodynamics (PD) • dose estimations

7. New Drug Characteristics

8. Properties of Drug CandidateDetermined by Neuroimaging • Whether it crosses the blood–brain barrier • And, thus, it is delivered to the target compartment (ie the brain) • Whether it engages the appropriate target • Receptor • Transporter • Enzyme • in a dose/exposure-related manner

9. Uses of PET in Drug Development • Two main approaches • Radiolabel the new drug • Estimate target occupancy • By the new drug

10. Drug Candidate Biodistribution and Kinetics Usual Radioisotopes: PET: [11C], [18F] SPECT: [123I], [99mTc] • To determine brain distribution of the compound • Some of its washout characteristics • Whether it is a substrate for BBB pumps • More likely to induce multidrug resistance • Also helpful in animal studies Ideally, a new lead drug candidate should be isotopically radiolabeled with a PET or SPECT radioisotope

11. Limitations to Radiolabeling Candidate Drugs • Primary quantitative radiotracers label only a very small number of potential drugs • Brain exposure for compounds that cross the BBB slowly may be underestimated • Given the short acquisition times possible with these isotopes • 90 min 11C • 8 h for 18F • Higher brain concentrations after chronic dosing • True target engagement may be overestimated • Total regional brain activity is recorded, not distinguishing between • binding to the test drug • brain-penetrated radiolabeled metabolites of the test drug • free tracer • tracer nonspecific binding

12. Instead of Radiolabeling Candidate Drug: Label Drug Target • Use existing PET/SPECT tracers • E.g., to the serotonin transporter • to determine the target occupancy of the new drug • in displacement studies • More active drug • Less labeled target available

13. Serotonin Trasporter Occupancy After Rx with Paroxetine • Studied with 123I-ADAM SPECT Decrease in midbrain uptake (71% serotonin transporter occupancy) Baseline After 6-week paroxetine 20 mg/day Catafau AM et al. Psychopharmacology 2006;189:145

14. PET versus SPECT for the Study of D2 Receptor Occupancy • Comparing: • [123I]IBZM SPECT • [11C]Raclopride PET • In the same subjects • Occupancy values measured by SPECT were lower than those measured with PET • 12.4% (occipital cortex as reference region) • 13.8% (cerebellum as reference region) • But the correlation between D2 occupancy using either method approximates 1.0 Catafau A et al. Neuroimage 2009, Epub ahead of print

15. PET: Better Anatomical Definition than SPECT (D2 Receptor Occupancy Study)

16. Functional MRI (fMRI) Arterial spin labeling (ASL) • To study • brain systems responses to external stimuli • the modification of these responses by drug treatment • In normal healthy volunteers • In patients • Complement neuroreceptor imaging by revealing the neurocircuitry involved in behavior and responses

17. Fearful Left Right Happy Left Right fMRI as Pre-Biomarker • The SSRI antidepressant citalopram • reduced amygdala activation in response to fearful faces in normal volunteers • The amygdala response to fearful stimuli • could develop into a pre-biomarker for antidepressant effects Harmer CJ et al. Biol Psychiatry 2006;59:816

18. Role of MRI in Drug DistributionProton (1H) Spectroscopy (MRS) • Assessment of glutamate and GABA amino acids • Psychotropic drugs with GABAergic or glutamatergic mechanisms of action • eg acamprosate, for the Rx of alcohol dependence • 7Li-MRS has been used to measure lithium concentrations in patients' brains • and to relate it to plasma concentrations (Plenge et al, 1994) • 19F-MRS has been used to determine concentrations of fluorinated antipsychotics • fluphenazine and trifluoperazine (Durst et al, 1990) • And antidepressants such as fluoxetine (Karson et al, 1993) • Also contain a fluorine atom • perphenazine, risperidone, most of the butyrophenone antipsychotics, • the SSRIs fluvoxamine and paroxetine • several benzodiazepines and the benzodiazepine antagonist flumazenil

19. Effect of Cytidine on Bipolar Depression and on Brain Glutamate Levels Region Studied Anterior Cingulate Gyrus MR spectrum Yoon SJ et al. Neuropsychopharmacology 2009 [Epub ahead of print]

20. Cytidine Effect Linked to  Glutamate Studied with MR spectroscopy Scores on the Hamilton Depression Rating Score Yoon SJ et al. Neuropsychopharmacology 2009 [Epub ahead of print]

21. Biomarker • Characteristic that is • objectively measured and evaluated • as an indicator of • normal biologic processes, • pathogenic processes, or • pharmacological responses to a therapeutic intervention Frank R, Hargreaves R. Nat Rev Drug Discov 2003;2: 566

22. Types of Biomarkers • Type 0 • Tracks the natural course of the disease • Type 1 • Examines the effects of intervention along with the known mechanism of action of the drug • But without strict relationship to clinical outcome • Type 2 • Change in the biomarker is predictive of clinical outcome • At present, most imaging methods in psychiatry do not meet the biomarker status

23. ‘Emerging’ Biomarker or ‘Pre-biomarker’ • With dopamine D2 receptor PET imaging • A link was found between • Useful to • fortify Proof of Concept for D2 DA antagonists • establish a dose range to test for new, pure D2 antagonists Dopamine D2 receptor occupancy efficacy to treat delusions, hallucinations

24. Dopamine D2 Receptor Binding D2 binding in a healthy control • Measured with • [11C]raclopride • Most widely used • [11C]FLB 457 • Better for cortical D2 • But still a [11C] compound • [18F]fallypride • Better for cortex, [18F] • But scanning time about 4 hours [11C]FLB 457 PET Takahashi H et al. Biol Psychiatry 2006;59:919

25. Dopamine D2 Receptor Binding in Schizophrenia • Decreased D2 receptor binding in untreated schizophrenia • Thalamus • Dorsomedial nucleus • Striatum • Anterior cingulate • Amygdala • Temporal cortex Patients < Controls Buchsbaum MS et al. Schizophr Res 2006;85:232 ACNP 2008, Phoenix Az

26. Reduced D2 Receptor Binding: Greater Receptor Occupancy by Endogenous DA • Striatal dopamine (DA) production is increased in schizophrenia • Measured with • [18F]fluoro-DOPA PET •  dopamine causes • down-regulation of the D2 receptors •  stimulation of the D2 receptors • Hallucinations • Treated by D2 antagonists 18FDOPA striatal Ki values Controls Patients Meyer-Lindenberg A et al. Nat Neurosci 2002;5:267

27. DA D2 Receptor Antagonists in Schizophrenia • Drug: Haloperidol • D2 binding studied with [11C]raclopride • Effective: <80% occupancy • >80% occupancy: parkinsonism or akathisia Kapur S et al. Am J Psychiatry 2000;157: 514

28. PET* to Assess Drug Behavior • Straightforward to assess: • Neutral orthosteric site antagonists • Haloperidol • Considerable challenges in the assessment of target engagement (TE) and linking TE to efficacy for: • Agonists • Partial agonists • Aripiprazole • Inverse agonists • Allosteric modulators • Many new therapeutic approaches to psychiatric disorders use positive allosteric modulators • As a means to fine-tune the primary excitatory (Glu) and inhibitory (GABA) systems *In combination with structural MRI

29. Partial Agonists:Occupancy Studies of the Antipsychotic Aripiprazole • Conventional wisdom: • within a 'therapeutic window' of 65–80% striatal D2 receptor occupancy • D2 DA antagonists have antipsychotic efficacy • with minimal EPS side effects • But for aripiprazole, occupancies closer to 90 or 95% were needed for the therapeutic range of the drug • Because the likely mechanism of action of aripiprazole is partial agonist at D2 receptors • The original 'therapeutic window' of 65–80% receptor occupancy is valid for D2 DA antagonists only

30. Antidepressants and dosing • SSRIs have been shown to occupy 80% or more of the serotonin transporter (SERT) • at clinically used doses • Within this class of drugs, this seems to be independent of the specific SSRI Meyer JH, et al. Am J Psychiatry 2001;158:1843

31. Antidepressant Dosing and PET: The Clomipramine Paradox • The tricyclic antidepressant (TCA) clomipramine • occupies 80% of the SERT • at doses as low as 10 mg • at a plasma concentration of 1.42 ng/ml • Yet, clinically used doses are 50–150 mg per day • Therapeutic plasma concentrations range 175–450 ng/ml* • Is blockade of the SERT and the norepinephrine transporter (NET) the therapeutic principle of clomipramine (and of the TCAs in general)? • Or do TCAs behave completely different from SSRIs, due to their broad pharmacological actions at many different molecular targets? • How valid are the studies upon which therapeutic doses and plasma concentrations have been determined for clinical use of the TCAs over decades? *Baumann P et al. Pharmacopsychiatry 2004;37:243

32. Dosing and PET • A radiotracer/pre-biomarker that has been demonstrated to predict the biological effects of a certain class of compounds • might lose its validity for a drug • with a slight modification of its mechanism of action • even if it binds to the same target molecule

33. Imaging and Dose Finding for Clinical Trials: Rejecting New Drugs • If a dose that demonstrated adequate target engagement in humans • With a high degree of confidence • does not have efficacy in the clinical trial • Proof of concept can be rejected, and a drug target can be abandoned more quickly

34. Rejecting a Disease Mechanism: Role of PET • Aprepitant blocked the neurokinin 1 receptor • Visualized with [18F]SPA-RQ PET • But did not improve depression • No better than placebo on the Hamilton Depression Scale Aprepitant Rx After 160mg for 40 days Baseline r

35. Summary ConclusionsNeuroimaging in Drug Development* • Justification/rationale for a specific neurotransmitter receptor system as a target for therapeutic intervention • E.g., dopamine, serotonin, etc • Radiolabeling the potential therapeutic compound of interest to examine biodistribution and BBB penetration • Rational therapeutic dosing • to test efficacy efficiently in the target patient population • Mechanism of pharmacological action • how does the efficacious action might occur • during therapeutic doses • in the target patient populations *Especially as pertains to neurotransmitter and neuroreceptor imaging

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38. Imaging to Determine Level of Target Engagement (TE) • With isotopically labeled drug candidates to estimate total brain exposure • generally using either C-11 or F-18 tracers • Functional studies such as measurement of test drug effects on regional cerebral blood flow • (PET/SPECT or MRI) • A direct measure of TE employing a PET radioligand that can be used for measuring occupancy • E.g., DA D2/D3 receptor occupancy studies with [11C]raclopride.

39. Imaging to Determine Level of Target Engagement (TE) • It is challenging to determine what level of TE is needed for a novel mechanism for a class of compounds • It is typically guided by efficacy studies in appropriate preclinical models

40. Target Identification and Therapeutic Rationale Example: Hyperdopaminergic state associated with the positive symptoms of schizophrenia • Studies of presynaptic DA neuron function measuring dopa decarboxylase with [18F]fluorodopa • Studies demonstrating elevations in amphetamine-induced intrasynaptic DA release • DA occupancy of D2/3 receptors by endogenous DA • Studies showing elevation of D2 receptor density or binding potential

41. Serotonin Trasporter Occupancy After Rx with Paroxetine • Relationship between • Paroxetine plasma concentrations • Serotonin transporter occupancy (%SERTocc) • Measured by means of 123I-ADAM SPECT Paroxetine plasma levels (ng/mL) Catafau AM et al. Psychopharmacology 2006;189:145

42. Dopamine antagonists (antipsychotics) • It has been known for more than 30 years that these compounds exert their effects on the positive symptoms of schizophrenia by antagonizing DA D2 receptors (Seeman et al, 1976). It was later discovered with PET that clinically effective doses of typical neuroleptics occupy D2-like DA receptors in the human striatum in the range between 65 and 90% (Farde et al, 1992). The suggestion of a 'therapeutic window' between 60 and 80% striatal D2 RO for sufficient treatment response and a 'ceiling' of around 80% occupancy, above which extrapyramidal side effects (EPS) are likely, was later confirmed by several other researchers (eg Kapur et al, 2000). Although clozapine and quetiapine seem to be exceptions, this rule also applies for most of the second-generation, 'atypical' antipsychotics (Nyberg et al, 1999). When their doses are raised above a certain threshold, striatal (and potentially extrastriatal) D2 DA occupancy increases to levels that are associated with a higher incidence of EPS. As described above the relationship between doses of antipsychotic drugs and their (striatal) D2-like DA RO has almost approached the status of a biomarker,

43. Dopamine antagonists (antipsychotics) • But long-term functional outcome in schizophrenia is determined by • improvement in cognitive function • rather than control of positive symptoms (Bowie et al, 2006) • And the available antipsychotics have limited activity against cognitive symptoms (Keefe et al, 2007) • Thus, D2/D3 receptor occupancy is an incomplete marker of disease control or progression Bowie CR et al. Am J Psychiatry 2006;163:425 Keefe RD et al. Arch Gen Psychiatry 2007;64:633

44. Target Identification and Therapeutic Rationale Example: Hypodopaminergic state associated with the abuse of cocaine, alcohol, ecstasy… • Furthermore, other evidences from studies in, as well as methamphetamine and ecstasy users, have demonstrated reductions of DA and some serotonin chemical markers supporting a concept of reduced DA function. **

45. Single vs Multiple Dosing of the Candidate Drug • In many cases single-dose studies very accurately predict receptor occupancies achieved with multiple dosing • But this might not be the situation for certain drugs • E.g., ziprasidone • Single-dose occupancy studies should be supplemented by subchronic studies • to avoid an incorrect dose selection for large efficacy trials

46. Single vs Multiple Dosing of Ziprasidone • For occupancy studies, ziprasidone was given to healthy volunteers in single oral doses of 40-60 mg • Causing a striatal D2 occupancy of 67%-85% • It was concluded from these studies that effective antipsychotic ziprasidone doses should be around 40 mg • But phase II clinical efficacy studies demonstrated 40 mg to be not better than placebo • Ziprasidone has potent antipsychotic efficacy at 120–160 mg • PET studies in patients after subchronic treatment demonstrated that the striatal D2 occupancy was markedly lower than was predicted by the early single-dose studies

47. Single vs Multiple Dosing of Ziprasidone Vernaleken I et al. J Clin Psychopharmacol 2008;28:608

48. Striatal vs Extrastriatal brain Occupancy • The selection of the correct brain region for determination of receptor occupancy • Is an increasingly crucial issue • Conventional belief of a 'therapeutic window' in the range of 60–80% striatal D2 occupancy • is true for most antipsychotics • But, low-affinity D2 antagonists • such as clozapine or quetiapine • occupy striatal D2 receptors (<50%) • to a lesser extent than cortical D2 receptors (>50%) (Gründer et al, 2006; Kessler et al, 2006). • A likely basis for their beneficial extrapyramidal side effect profile

49. Mechanism of Action of Candidate Therapeutic Drugs • PET imaging can provide actual empirical evidence for a drug candidate's effect or potential effect • E.g., potential use of DA transporter (DAT) inhibitors in the treatment of stimulant abuse • In nonhuman primates, the preadministration of the DAT inhibitor, GBR12909, blocked the amphetamine-induced DA release substantially • Before the amphetamine challenge, there was an increase of basal intrasynaptic DA, which could be useful in reducing, for example, cocaine craving