Download Evidence for the Existence of Nonmonotonic Dose

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
Amanita phalloides
Atropa belladonna
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.”
(1493 -1541)
Atropa belladonna
Philippus von Hohenheim (Paracelsus)
“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)
Improved contraction
(decreased arrhythmia)
EC50
(1493 -1541)
Cardiac Gycosides: e.g. digoxin
Na+,K+ ATPase inhibitors used to treat heart failure
Basic Principles
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”
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
BINDING
Concentration Response Curves
log transformed dose makes the “sigmoidal C/R curve”
Principals of Pharmacology:
“Receptor Occupancy Theory”- Critical Assumptions
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-activator 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
Nuclear Hormone Receptors:
One Receptor – Different Responses to Different Ligand
Allosteric Modulation by Different ligands
Different ligands (e.g. EDC) alter conformation to
change co-regulatory interactions
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
E2 & BPA Rapidly Activate ERK Signaling via ERb in
Developing and Mature Cerebellar Neurons
BPA (M) 10’
A
V10’
10–12
10-10
2
3
10-8
10-6
4
5
10-4
pERK1
pERK2
ERK1
ERK2
1
6
B
#
5
***
***
**
**
4
**
3
ERK1
ERK2
***
2
1
-8
-6
-4
10
10
10
-1
0
10
-1
2
10
le
0
Ve
hi
c
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
ERK Phosphorylation
(Fold Induction)
6
Bisphenol A (M) 10'
E2 & BPA Rapidly Activate ERK Signaling in Developing
Cerebellum of Rats
E2 - PND 8
BPA - PND 10
Rapid E2 and BPA signaling involves a high affinity
stimulatory and a lower affinity inhibitory binding sites
EC50 = 8 pM
EC50 = 0.4 nM
Zsarnovszky et al., 2005 Endocrinology
Concentration-Response Analysis of E2 and BPA
Effects on Contractility in Female Rat Myocytes
Belcher et al., 2012 Endocrinology
“Non-Monotonic Dose Responses”
Do They Exist?
YES – not limited to EDCs
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 nonselective or have variably selectivity for different receptors
• Not pre-screened for a receptor specific activity
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?