Principles of Pharmacology: A Foundation for Discussion on Low-Dose Effects & Non-Monotonic Dose Responses

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1 Principles of Pharmacology: A Foundation for Discussion on Low-Dose Effects & Non-Monotonic Dose Responses Scott M. Belcher, PhD University of Cincinnati Department of Pharmacology and Cell Biophysics

2 Goal: Non-Monotonic Concentration Response Relationships are Compatible with Receptor Mediated Responses We Accept Every Day U-Shaped Dose Response Curves for Essential Nutrients Toxic Effects Only at Low and High Concentrations

3 Modern Toxicology: The Dose Makes the Poison Quintal Responses: e.g. alive or dead yields sigmoidal C/R curve Atropa belladonna Amanita phalloides Atropa belladonna

4 Paracelsus: a Clinical Pharmacologist who presumed non-monotonic responses Alle Dinge sind Gift und nichts ist ohne Gift, allein die Dosis macht es, dass ein Ding kein Gift ist. ( ) All things are poison and nothing is without poison, only the dose permits something not to be poison. Philippus von Hohenheim (Paracelsus) Atropa belladonna

5 Alle Dinge sind Gift und nichts ist ohne Gift, allein die Dosis macht es, dass ein Ding kein Gift ist. All things are poison and nothing is without poison, only the dose permits something not to be poison. Philippus von Hohenheim (Paracelsus) ( ) Cardiac Gycosides: e.g. digoxin Na +,K + ATPase inhibitors used to treat heart failure

6 Principles of Pharmacology: Pharmacology: Pharmacodynamics: biochemical and physiological effects of drugs (EDCs) and mechanisms of actions What the agent does to the body Pharmacokinetics: process of drug (EDC) absorption, distribution, metabolism, elimination What the body does to the compound

7 Receptor Occupancy Theory of Drug (Toxic) Actions Eq. 1: D + R DR Effect Eq. 2b: K d = k -1 /k 1 = [D][R]/[DR] Eq. 3b: B = B max [D]/K d + [D] Eq. 2e: EC 50 = k -1 /k 1 = [D][R]/[DR] Eq. 3e: E = E max [D]/EC 50 + [D] BINDING EFFECT

8 Receptor Occupancy Theory of Drug (Toxic) Actions Eq. 1: D + R DR Effect log transformed of dose makes a sigmoidal C/R curve

9 Principles of Pharmacology: Receptor Occupancy Theory - Critical Assumptions

10 Principals of Pharmacology: Agonists Binding of different ligands at a receptor can have different responses Receptors are at an equilibrium between an active and inactive conformation Full Agonists: bind the active conformation of the receptor shifting the conformational equilibrium toward right Partial Agonist: bind both inactive and active conformation preferentially binds the active conformation Inactive Compound: non-selectively bind inactive and active conformations

11 Principals of Pharmacology: Agonists Binding of different ligands at a receptor can have different responses Receptors are at an equilibrium between an active and inactive conformation Inverse agonists: selectively bind the inactive form of the receptor and shift the equilibrium toward the inactive state Receptor systems with no basal activity: inverse agonists resemble competitive agonists Receptor systems with agonist-independent constitutive activity: inverse agonists inhibit basal activity Nuclear Hormone Receptors with basal activity: CAR; ERα p-ser 118; ERRγ, others Inverse agonists: ligands that block ligand-independent AF1 activity

12 Nuclear Hormone Receptors: Mechanisms of Hormonal, Drug, and EDC Actions & Receptor Theory Assumptions Broken One Receptor: Multiple Responses to Endogenous Hormone Ligand binding induces a conformation that allows a specific HR/co-regulator interaction Ligand and HRE are allosteric modulators that impact receptor interactions with specific co-regulator proteins Ligand bound HR can activate, repress or have no effect on expression of different hormone responsive genes depending on: 1) the nature of specific HRE 2) cell specific expression of co-regulators

13 Nuclear Hormone Receptors: Mechanisms of Hormonal, Drug, and EDC Actions & Receptor Theory Assumptions Broken One Receptor: Different Responses to Different Ligand Allosteric Modulation by Different Ligands Endogenous Hormone Different Ligand: hormone, drug or EDC Different or Opposite Response Different ligands (e.g. EDC) alter conformation to change co-regulatory interactions

14 Nuclear Hormone Receptors: ligand independent activity & heterodimers Endogenous Hormone Different Ligand Ligand-Independent Activity Heterodimers can alter response to different ligands Inverse Agonist Effects: inhibition of ligand-independent activity Indirect EDC Effects: e.g. inhibition of ligand-independent activity by increasing phosphatase activities

15 Principles of Pharmacology: Receptor Occupancy Theory - Critical Assumptions Assumptions are not valid in relation to mechanisms of hormone actions at Nuclear Receptors

16 A Clinically Important Example of Hormonal System Complexity Resulting in a Non-Monotonic Dose Response Curve GNRH Agonists: hrgnrh (gonadorelin) or synthetic analogs (e.g. leuprolide) are used clinically to inhibit LH & FSH release causing gonadal suppression Indications: Prostate cancer, advanced breast or ovarian cancer IVF controlled ovarian hyperstimulation Preciouses puberty Uterine fibroids Endometriosis

17 Principles of Pharmacology & Low-Dose Effects & Non- Monotonic Dose Responses Key Points, Considerations and Comments: Pharmacologically relevant low dose and non-monotonic concentration/response relationships exist Examples are well accepted for both therapeutic and toxic actions of natural and synthetic compounds Low dose and non-monotonic curves do not violate fundamental understanding of receptor mediated actions Complex biological systems do violate the assumptions necessary for receptor occupancy theory to accurately describe the concentration response relationships for many drug, natural and synthetic compounds Many natural or synthetic compounds (i.e. EDCs) are likely nonselective or have variably selectivity for different receptors (not prescreened for a receptor specific activity)

18 Key Points, Considerations and Comments: Why are low-dose and non-monotonic effect underappreciated? The pharmacokinetic properties of many compounds (absorption/distribution/biotransformation/elimination ) may dominate and mask pharmacodynamic properties Complex physiological feed back effects, with a tendency to retain equilibrium, may similarly impact detection Most studies are not concerned with such effects: Therapeutic leads are pre-selected Those effects are likely considered to have undesirable side-effects and compounds will not pass therapeutic screening Toxicity Assessments: such effects will be likely considered spurious, not dose responsive and not toxicologically relevant

19 Specific Considerations and Recommendations: The goals of regulatory toxicity studies often do not include establishing dose response relationships Most studies do not have enough doses characterize the dose response relationships and doses used are usually too widely spaced Recommendation: For regulatory toxicity studies, the identification of significant effects for specific endpoints at intermediate or low concentrations should warrant additional and focused investigation of those effects Assessment may be limited to identified endpoints and physiologically related and informative endpoints Assessment would include testing of additional doses with the specific goal of characterizing the dose response relationship of observed effects