Evidence for the Existence of Non-monotonic Dose-response ...
Transcript of Evidence for the Existence of Non-monotonic Dose-response ...
Evidence for the Existence of Non-monotonic Dose-response: Does it or Doesn’t it?
Scott M. Belcher, PhD
University of Cincinnati
Department of Pharmacology and Cell Biophysics
Evidence for the Existence of Non-monotonic Dose-response: Does it or Doesn’t it?
Answer: Yes
Every day we accept :
The U-Shaped Dose Response Curve for Essential Nutrients Toxic Effects at Low and High Concentrations
Modern Toxicology: “The Dose Makes the Poison”
Quintal Responses: e.g. alive vs. dead; tumor vs. tumor “sigmoidal C/R curve”
Atropa belladonna
Atropa belladonna
Amanita phalloides
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
(1493 -1541)
“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)
(1493 -1541)
Cardiac Gycosides: e.g. digoxin
Na+,K+ ATPase inhibitors used to treat heart failure
Improved contraction
(decreased arrhythmia)
EC50
Pharmacology/Toxicology: 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”
Basic Principles
BINDING
Receptor Occupancy Theory of Drug (EDC) Action
Eq. 1: D + R ↔ DR → Effect
Eq. 2: EC50= k-1/k1 = [D][R]/[DR] Eq. 2b: Kd = k-1/k1 = [D][R]/[DR]
Eq. 3: E = Emax [D]/EC50 + [D] Eq. 3b: B = Bmax [D]/Kd + [D]
EFFECT
Nuclear Hormone Receptors: Mechanisms of Hormonal, Drug, and EDC Actions & Receptor Theory Assumptions Broken
One Receptor – Multiple Responses to Endogenous Hormone
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
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-activator proteins
Different ligands (e.g. EDC) alter conformation to change co-regulatory interactions
Nuclear Hormone Receptors:
One Receptor – Different Responses to Different Ligand Allosteric Modulation by Different ligands
Principals of Pharmacology: “Receptor Occupancy Theory”- Critical Assumptions
Assumptions are not valid in relation to mechanisms of hormone actions at Nuclear Receptors
?
Steroid Hormones Signal Through Nuclear Hormone Receptors and Intracellular Signaling Pathways:
Rapid Estrogen Signaling
Rapid Signaling Mechanism in Cerebellar Neurons
Wong et al., 2003 J. Neurosci.; Belcher et al. 2005 Endocrinology
Vehic
le -12
10-1
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10-8
10-6
10-4
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ERK1ERK2
Bisphenol A (M) 10'
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BPA (M) 10’
V10’ 10–12 10-10 10-8 10-6 10-4
pERK1
pERK2
ERK1
ERK2
1 2 3 4 5 6
BPA (M) 10’
V10’ 10–12 10-10 10-8 10-6 10-4
pERK1
pERK2
ERK1
ERK2
1 2 3 4 5 6
A
B
Vehic
le -12
10-1
0
10-8
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ERK1ERK2
Bisphenol A (M) 10'
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*****
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#
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K P
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(Fo
ld In
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BPA (M) 10’
V10’ 10–12 10-10 10-8 10-6 10-4
pERK1
pERK2
ERK1
ERK2
1 2 3 4 5 6
BPA (M) 10’
V10’ 10–12 10-10 10-8 10-6 10-4
pERK1
pERK2
ERK1
ERK2
1 2 3 4 5 6
A
B
E2 & BPA Rapidly Activate ERK Signaling via ERb in Developing and Mature Cerebellar Neurons
Jakab et al., 2001 JCN; Wong et al., 2001 J. Neurosci Methods; Wong et al., 2003 J. Neurosci.; Belcher et al. & Zsarnovszky et al., 2005 Endocrinology
EC50 = 8 pM
EC50 = 0.4 nM
Zsarnovszky et al., 2005 Endocrinology
E2 & BPA Rapidly Activate ERK Signaling in Developing Cerebellum of Rats
Rapid E2 and BPA signaling involves a high affinity
stimulatory and a lower affinity inhibitory binding sites E2 - PND 8 BPA - PND 10
Concentration-Response Analysis of E2 and BPA Effects on Contractility in Female Rat Myocytes
Belcher et al., 2012 Endocrinology
Key Points, Considerations and Comments:
• Pharmacologically relevant “non-monotonic” concentration/response relationships exist
• Examples are well accepted for both therapeutic and toxic actions of natural and synthetic compounds
• “Non-monotonic” curves do not violate fundamental understanding of receptor mediated actions
• All 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 non-selective or have variably selectivity for different receptors
• Not pre-screened for a receptor specific activity
“Non-Monotonic Dose Responses” Do They Exist?
YES – not limited to EDCs
Key Points, Considerations and Comments: Why are non-monotonic effect “underappreciated”
• Most studies are not concerned with such effects:
• Toxicity Assessments: The goals of regulatory toxicity studies (e.g. standard multigenerational) do not include establishing dose response relationships
• Therapeutics: “leads” are pre-selected, such effects are likely considered as undesirable side-effects and compounds will not pass therapeutic screening
• Pharmacokinetic properties (ADME) of many compounds may dominate or mask some pharmacodynamic properties
• Complex physiological feed-back effects and interactions between “systems” may impact detection
Q: How Does Toxicology and RA Include Complex Dose Response Relationships in the Decision Process?