17.12: Dose Response Curve: Conventional Versus Nonmonotonic

The correlation between a drug's dosage and its impact on a biological system is a cornerstone of pharmacology and toxicology. Conventional dose–response curves, which include graded and quantal relationships, are key to this understanding. Graded dose–response curves depict the spectrum of a biological reaction to different doses within an individual, indicating that as the drug dosage increases, so does the intensity of the response. On the other hand, quantal dose–response relationships illustrate the binary responses observed across a population, where the effect is either present or absent.

For evaluating drug safety, the quantal dose–response curve is instrumental in calculating the therapeutic index (TI). TI is a critical measure of the ratio of the dose that causes toxicity (LD₅₀) to the dose that elicits the desired therapeutic effect in 50% of the population (ED₅₀). A higher TI indicates a safer drug. The safety margin, provided by LD₁ and ED₉₉ values, ensures safety even for the most vulnerable individuals.

Nonmonotonic dose–response curves, such as U-shaped, hockey stick-shaped, and inverted U-shaped, represent more complex relationships between dose and response. A U-shaped curve can indicate a deficiency at low doses and toxicity at high doses. Essential metals and vitamins mainly follow it. A hockey stick-shaped curve reflects a region of no response followed by a steep increase in response, a pattern seen with certain protective mechanisms involved in detoxifying toxicants like formaldehyde. In contrast, an inverted U-shaped curve occurs when an initial positive response to a drug diminishes or reverses at higher doses due to receptor downregulation or desensitization, as observed with certain hormonal therapies.

These nonmonotonic patterns signify the intricate nature of biological systems and underscore the importance of a comprehensive assessment of a drug's effects across various doses. Recognizing these diverse dose–response relationships is crucial for accurately evaluating drug efficacy and safety.

Conventional dose-response curves are sigmoidal, including individual graded and population-wide quantal relationships.

The quantal dose–response curves for the therapeutic effect and lethality show ED₅₀ and LD₅₀, respectively, together generating the therapeutic index, TI.

Drugs with high TI are generally safe unless the patient is allergic. However, lower TI drugs should be used cautiously.

Alternatively, comparing the ED₉₉ and LD₁ yields a margin of safety.

Nonmonotonic dose-response curves can be U-shaped, hockey stick-shaped, and inverted U-shaped.

The U-shaped curve’s bottom represents the optimal dose achieving homeostasis, with overdose toxicity and increased death risk above it.

Toxicants can saturate detoxifying mechanisms, leading to an initial no-response followed by a sharp rise in an adverse response, displaying a hockey stick-shaped plot.

Prolonged drug exposure desensitizes receptors, showing an initial rise to maximal response, followed by a decline at extremely high levels, forming an inverted U-shaped curve.