A prodrug, also known as a precursor drug, generally refers to a class of compounds that are themselves inactive or have weak activity but, after conversion within a biological organism, produce metabolites or parent drugs with significantly enhanced pharmacological activity. The design and development of prodrugs are common and effective strategies in the drug screening and lead compound optimization process. Typically, modifications are made to the most suitable functional groups in the parent drug molecule to improve its physicochemical properties and overcome adverse properties such as poor absorption, distribution, metabolism, excretion, and toxicity.
Prodrugs and Their Bioactivation
In most cases, based on known biological transformations, corresponding structural designs are made on the basis of the parent drug, and the parent drug is released and exerts its pharmacological effect through enzymatic or non-enzymatic activation. Depending on the structure of the prodrug and the mechanism of activation, prodrugs can be divided into two categories: carrier prodrugs and bioprecursor prodrugs.
- Carrier Prodrugs
Carrier prodrugs contain non-toxic carrier groups or precursor groups that are covalently attached to the parent drug molecule. Some common functional groups in parent drug molecules include hydroxyl, amino, carboxyl, thiol, and aldehyde groups, among others. After attachment to the carrier group, ester bonds, carbonate salts, amides, phosphate salts, and aminoformates, among other functional group structures, are formed.
Esters are the most widely used in carrier prodrug design. Ester bonds can be hydrolyzed by esterases found in blood, liver, and other organs or tissues, releasing the drug from its prodrug precursor molecule. Common ester-type prodrugs include ethyl esters, aryl esters, diol esters, cyclic carbonate esters, and lactones. Telotristat etiprate, an inhibitor of tryptophan hydroxylase, is an example of an ester-type prodrug. After oral administration in humans, the ethyl functional group in its structure is hydrolyzed by carboxylesterases in the body, rapidly converting it to the active drug telotristat, with minimal exposure to telotristat prodrug in the systemic circulation.
Some simple alkyl esters or aryl esters may not be efficiently hydrolyzed by esterases or have low enzymatic hydrolysis efficiency, resulting in inadequate systemic exposure. In such cases, by introducing a double ester group, the prodrug molecule can be more easily recognized and activated through a second esterase site. The bisester prodrug isavuconazonium sulfate, when administered intravenously, is rapidly hydrolyzed by plasma esterases at the amino acid ester bond site, while the released hydroxyl attacks another ester bond site within the molecule, leading to intramolecular cyclization, producing the active drug isavuconazole, the inactive cyclic byproduct (BAL8728), and acetaldehyde.
Amides can be designed as prodrugs to enhance oral absorption and targeted transport and are hydrolyzed by intracellular proteases or peptidases to release active drugs. Amino acid prodrug LY544344, utilizing the high affinity of the internal proline group for intestinal epithelial cell oligopeptide transporter PepT1, is transported into cells and is enzymatically hydrolyzed into LY35470 and then passively diffuses into the bloodstream. Prodrugs characterized by the amine functional group also include sulfonyl amides and aminoformates, which can be effectively converted by esterases or amidases into active drugs.
In addition to enzyme-mediated activation, there are carrier prodrugs that undergo non-enzyme-mediated activation under specific physiological conditions, such as hydrazone prodrugs and N-Mannich base prodrugs. Hydrazones can stably exist within the normal pH range of the systemic circulation but undergo cleavage and release of the parent drug in acidic tissue environments or intracellular compartments. N-Mannich bases decompose in neutral and alkaline environments, triggering the release of the parent drug as pH increases. For example, the highly water-soluble rolitetracycline converts to the parent drug tetracycline at pH 7.4.
- Bioprecursor Prodrugs
Bioprecursor prodrugs do not contain carrier groups and only become active compounds after metabolism by biotransforming enzymes (including oxidation, reduction, and conjugation), such as the anti-inflammatory drug sulindac, which can be reversibly reduced to its thiol form under the action of metabolic enzymes, and the antihypertensive drug losartan, which can be considered a bioprecursor prodrug that is oxidized in the body to its carboxylic acid active metabolite to exert its effects.
Prodrug Pharmacokinetic Property Improvement
Prodrugs create new chemical entities based on the parent drug molecule and often come with changes in drug properties. Prodrug design and optimization can effectively address pharmacokinetic property deficiencies of the parent drug, such as poor bioavailability due to low lipid solubility or poor water solubility, low efficacy due to rapid metabolism, or drug toxicity due to insufficient targeting.
- Improving Drug Water Solubility
The prerequisite for drug absorption is that it should be in a dissolved state, and poor water solubility hinders drug absorption and transport in the body. Using prodrug strategies to introduce polar functional groups such as amino acid esters, polyethylene glycols, sugars, or phosphate esters into the drug structure can significantly increase drug solubility. Degoey et al. designed a phosphate prodrug of lopinavir, which had a water solubility more than 700 times higher than free lopinavir. Pharmacokinetic studies showed that after oral administration, the levels of active drug released from the phosphate prodrug in the bloodstream were higher.
- Impact on Drug Membrane Permeability
The ability of a drug to penetrate cell membranes is related to its lipophilicity and recognition by transport proteins on the membrane. Increasing drug lipophilicity by shielding polar groups or introducing groups recognized by transport proteins can enhance drug membrane penetration. For zanamivir, a neuraminidase inhibitor, its polar structure results in very low oral bioavailability. However, the amino acid ester prodrug can be recognized by PepT1, showing better intestinal membrane permeability and bioavailability. Remdesivir, the first FDA-approved treatment for COVID-19, utilizes a phosphoramidate prodrug design, incorporating aryl ester, proline, and alkoxy ester groups to improve its cell membrane permeability. It undergoes intracellular metabolism to form the active triphosphate compound within cells, where it accumulates and interferes with viral RNA transcription.
- Enhancing Drug Metabolic Stability
Metabolic instability can reduce the total amount of active drug entering the systemic circulation, making it less effective. Prodrug strategies can be used to modify specific functional groups to shield vulnerable metabolic sites, thereby improving drug metabolic stability and protecting the active drug from systemic metabolism. For example, the bronchodilator bambuterol is a prodrug of terbutaline, designed as a 2-methylaminoethyl ester. Due to the protection of the phenolic hydroxyl group in the drug’s structure, its first-pass metabolism is significantly reduced. Bambuterol is primarily metabolized to terbutaline through slow, non-specific acetylcholinesterase activation.
- Enhancing Drug Targeting
The design of targeted prodrugs often involves coupling structural units that target specific tissues or cells with cytotoxic molecules. For example, peptide-drug conjugate (PDC) prodrugs covalently conjugate peptides to drugs using different linkers, allowing them to selectively accumulate in tumor cells, extend half-life, and enhance therapeutic efficacy. Antibody-drug conjugates (ADCs) are another example of carrier prodrugs, utilizing monoclonal antibodies with high affinity for surface antigen proteins on tumor cells. These ADCs target tumor cells through receptor-mediated endocytosis, leading to the release of active drugs.
Preclinical Research on Prodrugs
The design of prodrugs must ensure their ability to degrade and release the parent drug at the appropriate time and site. This requires that the designed prodrugs have suitable chemical stability and enzymatic stability. They should neither degrade before reaching the target site nor remain undegraded for an extended period after entering the target site, as this would hinder their pharmacological effect. Additionally, prodrugs often have distinct chemical structures from the parent drug, which may introduce new toxicities. On one hand, this could result from unexpected metabolism of the prodrug, leading to the formation of unforeseen metabolites. On the other hand, it may occur because inert carriers generated from prodrug cleavage can transform into toxic metabolites, such as formaldehyde and acetaldehyde. Therefore, the comprehensive evaluation of prodrugs should include their physicochemical properties, pharmacological effects, pharmacokinetic properties, and potential toxic reactions.
References:
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