Pharmacodynamics: Complex Concentration-Response Relationships and Hysteresis

Apparent Dissociation in Concentration-Effect Relationships

There are instances in pharmacodynamics where it appears that there is no relationship between drug concentration and effect.
This perception often occurs when looking at parent drug concentrations in the plasma, leading to a complete dissociation of the concentration-effect relationship.
Gaining mechanistic insight and looking deeper into the details often reveals that a relationship does exist; it is simply not immediately obvious.
One category of response that lacks a standard concentration relationship includes anaphylactic reactions. These require an initial priming exposure followed by a second exposure that triggers an overwhelming response. This lecture excludes these and focuses on situations where an obvious relationship exists but is hidden from plain view.

Simple Explanations: The Extremes of the Curve

One of the simplest reasons for seeing no change in response despite changes in concentration is being at either end of the concentration-effect spectrum.
The Bottom End of the Curve: At very low concentrations, the drug may elicit minimal to no effect. * In this region, a tenfold ( $10 \times$ ) change in concentration might result in no observable change in response because the threshold for activity has not been reached.
The Top End of the Curve (Asymptote): At very high concentrations, the drug has reached its maximal effect ( $E_{max}$ ). * Similarly, a tenfold ( $10 \times$ ) change in concentration here will result in almost no change in effect because the system is saturated at the top asymptote of the concentration-effect relationship. * In this scenario, a large effect would already be present, making it obvious which end of the spectrum the drug is at.

Counterclockwise Hysteresis and Distribution Delays

Confusing data can present as massive differences in response for the same plasma concentration, or wildly different drug concentrations resulting in the same response.
This confusion often stems from the simplistic view that drug action follows targets site concentration instantaneously and that plasma concentration is an exact surrogate for tissue concentration.
While drug in the tissue and plasma are often in free equilibrium (moving up and down together), this is not true for all tissues.
Equilibration Rates in Different Tissues: * Liver: Often equilibrates with drug in the plasma very rapidly, mirroring plasma concentrations closely. * Muscle: For certain drugs like Thiopental, the concentrations do not reach an instantaneous equilibrium; it takes time to distribute into the muscle tissue. * Adipose Tissue (Fat): May take many hours after a dose is administered to reach equilibrium.
The Blood-Brain Barrier (BBB) and CNS Drugs: * Sedative drugs intended for the Central Nervous System (CNS) must cross the blood-brain barrier into the Cerebrospinal Fluid (CSF). * Thiopental: Equilibrates relatively rapidly across the blood-brain barrier. * Phenobarbital and Barbital: Equilibrate at a slower rate than Thiopental. * Salicylic Acid: Equilibrates very slowly. This is because it is a very polar molecule and is highly ionized at physiological pH due to its acidic nature.
"Spy Movie" Myth: The trope of a person falling asleep instantaneously after a needle injection is often inaccurate. Even with intravenous administration, the drug must equilibrate in the CNS and reach high enough concentrations to elicit a response, which takes time.
Succinylcholine Example: Even when dosed intravenously, there is a delay between the injection into venous circulation and the effect on muscle tissue. Although it distributes rapidly once it arrives, the travel and initial distribution time create a lag in the onset of effect.

Temporal Relationships and Sequential Data

When concentration-effect data points are connected in chronological sequence (e.g., 1 hour, 2 hours, 3 hours, etc.), a pattern known as a hysteresis loop emerges.
Counterclockwise Hysteresis: The data forms a loop moving in a counterclockwise direction over time. * Early Phase: High plasma concentrations are present, but there is minimal effect because the drug has not yet distributed to the effect site. * Late Phase: Plasma concentrations have dropped to low levels, but the effect is at its maximum because the drug concentrations at the effect site (target tissue) are high due to slow redistribution/clearance from that tissue.
This illustrates that response is dependent on the time post-dose, not just the current plasma concentration.

Cascades of Events and Complex Physiology

A delay in response relative to plasma concentration is not always caused by slow distribution; it can also be caused by a complex cascade of physiological events.
Ibuprofen and Fever: * Ibuprofen is used to lower core body temperature (fever) by inhibiting cyclooxygenase (COX). * The core body temperature does not drop immediately with the rise of plasma concentration. * The delay is due to the process of altering the inflammatory cascade and resetting the body's temperature set point, which involves multiple biological steps.
Warfarin and Anticoagulation: * Warfarin concentrations may be very high early on, but the anticoagulant effect (measured via blood activity) is very low. * Conversely, when plasma concentrations of Warfarin are nearly negligible or zero, the maximal anticoagulant response is often observed. * Mechanism: Warfarin interferes with the enzyme Vitamin K oxidoreductase. The resulting change in enzyme function must then translate through the entire clotting cascade before the final anticoagulant action is observed. * Warfarin has a relatively short half-life, meaning the drug may be cleared from the plasma by the time its full physiological impact is realized.

Clockwise Hysteresis and Tolerance

Clockwise Hysteresis: If data points connected in time sequence show that early time points have a greater response for a given concentration than later time points, it is called clockwise hysteresis. * It can be viewed as requiring a higher concentration at later times to produce the same response seen at earlier times.
Pharmacological Tolerance: This is the decrease in response to the same concentration of a drug over time.
Mechanisms of Tolerance: While often poorly understood and sometimes appearing as physiological adaptations, common mechanisms include: 1. Receptor Downregulation: A decrease in the number of receptors, which may be internalized into the cellular membrane in response to constant stimulation. 2. Depletion of Chemical Mediators: Constant use depletes the mediators needed to translate the drug's effect into a cellular response. 3. Decoupling of Second Messenger Systems: The system disconnects the receptor activation from the cellular response to prevent over-activation. 4. Physiological Adaptation: For example, the body maintaining water balance in response to constant diuretic use.
Tolerance does not result in a delay of response, but a diminishing magnitude of response despite consistent exposure.

Active Metabolites and Routes of Administration

The presence of active metabolites can complicate the concentration-effect relationship of the parent drug.
Alprenolol Study: * Two data sets were compared: one following an oral dose and one following an intravenous (IV) dose. * Response Measured: Reduction in exercise-induced heart rate. * Observation: For any given concentration of the parent drug Alprenolol in the plasma, the oral dose produced a significantly greater response than the IV dose. * Apparent Potency: It appears that Alprenolol is more potent when given orally, as a much higher IV concentration is needed to produce the same effect.
Explanation: The molecules are not "magically" more potent via the oral route. Instead, first-pass metabolism after oral administration likely produces active metabolites that contribute to the therapeutic effect, whereas IV administration bypasses this initial metabolic phase, resulting in a lower overall effect for the same parent drug concentration.