• Beyond the effect compartment approach to handling distributional hysteresis, some drugs produce their effects through indirect mechanisms.

  • For indirect mechanisms the drug does not directly activate or inhibit a receptor to produce the observed response, but rather modulates a biological process such as affecting synthesis or degradation that then produces the effect.

    • For such drugs, even at full effect-site equilibration, there is an intrinsic temporal delay because the effect depends on the kinetics of an intermediary biological process.^4,6 

• Indirect response models describe four basic mechanisms

  • Model I

    • Inhibition of input (synthesis):

      • In this case, the drug inhibits production of a response variable (e.g., corticosteroids inhibiting cortisol synthesis, statins inhibiting cholesterol synthesis).

        • The response variable falls slowly as its ongoing production is suppressed while degradation continues.

    Model II

    • Stimulation of input:

      • Here the drug stimulates production of a response variable so the drug effect builds slowly as synthesis is enhanced.

    Model III

    • Inhibition of output (degradation)

      • In this model the drug inhibits degradation of a response variable, causing it to accumulate and the effect accumulates slowly.

    Model IV

    •  Stimulation of output

      • For Model IV the drug stimulates degradation, reducing the response variable.

    • The clinical relevance of indirect response models is apparent with warfarin.

      • Warfarin inhibits vitamin K epoxide reductase, thereby suppressing synthesis of clotting factors II, VII, IX, and X.

        • But the anticoagulant effect (prolongation of PT/INR) depends not just on synthesis suppression but on the rate of degradation of existing clotting factors, each with its own half-life (factor VII ~6 hours; prothrombin ~72 hours). [PT: Prothrombin Time; INR: International Normalized Ratio which describes how quickly blood clots]

        • These dependencies explain why warfarin full anticoagulant effect takes days to develop after initiation, and why the INR remains elevated for days after warfarin cessation. The the clotting factor synthesis has recovered, but the long-lived factors must be replaced through normal turnover.^1,2 

Pharmacodynamic Drug-Drug Interactions

• Pharmacodynamic drug-drug interactions occur when the pharmacological effect of one drug is altered by the presence of another, independently of any pharmacokinetic change in either drug's plasma concentration.

  • These interactions work at the level of the receptor, effector system, or downstream physiological response.^7,8 

• Three categories define these interactions:

  • Pharmacodynamic synergism

    • Pharmacological synergism occurs when the combined effect of two drugs exceeds what would be expected from simple addition of their individual effects.

      • Two formal frameworks define the additive reference against which synergism is measured:

        • Loewe additivity

          • Loewe additivity defines additivity by the principle that a drug cannot synergize with itself:

            • If drug A at dose X produces the same effect (equieffective doses) as drug B at dose Y, the combination of X/2 of A and Y/2 of B should produce the same effect.

            • Deviation from the straight isobole line (toward the origin) defines Loewe synergism.^7,8 

        • Bliss independence

          • Bliss independence defines additivity based on statistical independence of two drugs acting through different mechanisms: the combined fractional effect equals 1 − (1−E₁)(1−E₂).

          • This framework assumes mechanistic independence and is more appropriate when the two drugs act at genuinely different molecular targets.^7,8

      • Clinical examples and pharmacodynamic synergism

        • Trimethoprim + sulfamethoxazole, sequential blockade of the same bacterial folate synthesis pathway (Loewe synergism)

        • β-lactam + aminoglycoside antibiotic combinations

        • Antihypertensive combination regimens such as ACE inhibitor + calcium channel blocker are examples of complementary pathways producing greater BP reduction than either alone.

    • Pharmacodynamic additivity

      • The null interaction occurs when combined effects equal the sum of independent effects.

        • Many combination drug therapies are additive in their therapeutic effects, which is sufficient justification for combination therapy when lower doses of each component reduce individual adverse effects.

    • Pharmacodynamic antagonism

      • Pharmcodynamic antagonism occurs when one drug reduces the effect of another.

        • Antagonism is either competitive in which the antagonist occupies the agonist's receptor or functional when the two drugs act at separate receptors or pathways that oppose each other physiologically.

        • Examples of clinical importance

          • Naloxone functionally antagonizes opioid respiratory depression though competitive action at opioid receptors.

          • β₂-agonists (salbutamol) functionally antagonize bronchoconstriction produced by non-selective β-blockers, opposing effects on bronchial smooth muscle.

          • Flumazenil competitively antagonizes benzodiazepine sedation at GABA-A receptor benzodiazepine sites.

          • Vitamin K antagonizes warfarin's anticoagulant effect by restoring the vitamin K epoxide reductase substrate supply.^1,2