Thiazolidinediones

Mechanism of action and main targets

Thiazolidinedione drugs exert their effects at the intracellular level. Their main target is nuclear PPAR receptors (peroxisome proliferator-activated receptors). These structures are localized in the nuclei of insulin-dependent tissues, predominantly in fat cells (adipocytes). The primary function of PPAR receptors is to regulate the transcription of genes responsible for lipid and carbohydrate metabolism.

There are two main types of receptors affected by thiazolidinediones, each triggering specific effects:

• PPAR-$\gamma$ (gamma): regulate adipocyte differentiation processes, and tightly control adipogenesis and the accumulation of fatty acids in cells. A crucial effect of their activation is the stimulation of glucose transporter synthesis (GLUT-4) and ensuring their incorporation into cell membranes.
• PPAR-$\alpha$ (alpha): are responsible for regulating lipid metabolism, specifically fatty acid oxidation. In addition, they influence the metabolism of inflammatory mediators (prostaglandins) by stimulating the synthesis of the COX-2 enzyme.

Pharmacodynamic effects

The key effect of thiazolidinediones is aimed at combating insulin resistance. The drugs act as PPAR-$\gamma_2$ receptor agonists. As a result of their action, the population of small adipocytes increases, possessing significantly higher insulin sensitivity compared to large fat cells.

The effect on carbohydrate metabolism is realized through two parallel mechanisms:

1. An increase in the number of insulin-dependent glucose transporters (GLUT-4), which facilitates glucose transport into the cell.
2. A decrease in the concentration of free fatty acids and glycerol in blood plasma (under normal conditions, an excess of these substances prevents adequate glucose uptake by tissues).

In addition to metabolic effects, thiazolidinediones affect the vascular wall and slow down the progression of atherosclerosis. This mechanism is associated with antagonism toward PPAR-$\alpha$ receptors. As a result, the catabolism of inflammatory mediators is accelerated, the level of atherogenic lipoproteins is reduced, and simultaneously the level of anti-atherogenic high-density lipoproteins (HDL) in blood plasma is increased.

Pharmacokinetics and use of pioglitazone

The main representative of the group, according to its chemical structure, is pioglitazone (Pioglitazone, known trade names are Actos and Pioglar). Other members of this group include rosiglitazone (Rosiglitazone, Avandia) and troglitazone (Troglitazone).

The pharmacokinetic parameters of pioglitazone are as follows:

• Absorption: occurs rapidly; the drug enters the systemic circulation within 30 minutes after administration.
• Maximum concentration ($C_{max}$): is reached in the blood after 2 hours.
• Duration of action: therapeutically high concentration is maintained for approximately 24 hours.
• Metabolism: occurs in the liver with the mandatory participation of the cytochrome P450 system, resulting in the formation of two active metabolites.
• Excretion: unchanged pioglitazone is excreted via bile. Its metabolites leave the body through the intestines and kidneys (renal excretion accounts for 15–30%).
• Half-life ($t_{1/2}$): for pioglitazone itself is 3–7 hours, and for its active metabolites — from 16 to 24 hours.

Due to its long duration of action, the dosing regimen of pioglitazone is convenient — the drug is taken once a day. In clinical practice, it is used both as monotherapy and as part of combination therapy. Pioglitazone is successfully combined with sulfonylureas, biguanides, and insulin preparations.

Side effects

Despite its high efficacy, the use of thiazolidinediones is associated with the risk of certain adverse reactions. The main side effects include:

• Development of hypoglycemia (especially in combination therapy).
• Occurrence of edema.
• Anemia.
• Decrease in bilirubin concentration, as well as a drop in liver enzyme levels in the blood.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of regulation of adipocyte differentiation via PPAR-gamma receptors
• Effect of PPAR-alpha receptors on COX-2 synthesis and prostaglandin metabolism
• Features of pioglitazone metabolism involving the cytochrome P450 system
• The role of free fatty acids and glycerol in the development of tissue insulin resistance
• Comparative characteristics of pioglitazone, rosiglitazone, and troglitazone

Other topics in the section "Hormone preparations, their synthetic substitutes, and antagonists"

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