Prodrug


A prodrug is a medication or compound that, after administration, is metabolized into a pharmacologically active drug. Instead of administering a drug directly, a corresponding prodrug can be used to improve how the drug is absorbed, distributed, metabolized, and excreted.
Prodrugs are often designed to improve bioavailability when a drug itself is poorly absorbed from the gastrointestinal tract. A prodrug may be used to improve how selectively the drug interacts with cells or processes that are not its intended target. This reduces adverse or unintended effects of a drug, especially important in treatments like chemotherapy, which can have severe unintended and undesirable side effects.

History

Many herbal extracts historically used in medicine contain glycosides of the active agent, which are hydrolyzed in the intestines to release the active and more bioavailable aglycone. For example, salicin is a β-D-glucopyranoside that is cleaved by esterases to release salicylic acid. Aspirin, acetylsalicylic acid, first made by Felix Hoffmann at Bayer in 1897, is a synthetic prodrug of salicylic acid. However, in other cases, such as codeine and morphine, the administered drug is enzymatically activated to form sugar derivatives that are more active than the parent compound.
The first synthetic antimicrobial drug, arsphenamine, discovered in 1909 by Sahachiro Hata in the laboratory of Paul Ehrlich, is not toxic to bacteria until it has been converted to an active form by the body. Likewise, prontosil, the first sulfa drug, must be cleaved in the body to release the active molecule, sulfanilamide. Since that time, many other examples have been identified.
Terfenadine, the first non-sedating antihistamine, had to be withdrawn from the market because of the small risk of a serious side effect. However, terfenadine was discovered to be the prodrug of the active molecule, fexofenadine, which does not carry the same risks as the parent compound. Therefore, fexofenadine could be placed on the market as a safe replacement for the original drug. Loratadine, another non-sedating antihistamine, is the prodrug of desloratadine, which is largely responsible for the antihistaminergic effects of the parent compound. However, in this case the parent compound does not have the side effects associated with terfenadine, and so both loratadine and its active metabolite, desloratadine, are currently marketed.

Recent prodrugs

Approximately 10% of all marketed drugs worldwide can be considered prodrugs. Since 2008, at least 30 prodrugs have been approved by the FDA. Seven prodrugs were approved in 2015 and six in 2017. Examples of recently approved prodrugs are such as dabigatran etexilate, gabapentin enacarbil, sofosbuvir, tedizolid phosphate, isavuconazonium, aripiprazole lauroxil, selexipag, latanoprostene bunod, and benzhydrocodone.

Classification

Prodrugs can be classified into two major types, based on how the body converts the prodrug into the final active drug form:
Both major types can be further categorized into subtypes, based on factors such as whether the intracellular bioactivation location is also the site of therapeutic action, or whether or not bioactivation occurs in the gastrointestinal fluids or in the circulation system.
TypeBioactivation siteSubtypeTissue location of bioactivationExamples
Type IIntracellularType IATherapeutic target tissues/cellsAciclovir, fluorouracil, cyclophosphamide, diethylstilbestrol diphosphate, L-DOPA, mercaptopurine, mitomycin, zidovudine
Type IIntracellularType IBMetabolic tissues Carbamazepine, captopril, carisoprodol, heroin, molsidomine, leflunomide, paliperidone, phenacetin, primidone, psilocybin, sulindac, fursultiamine, codeine
Type IIExtracellularType IIAGI fluidsLoperamide oxide, oxyphenisatin, sulfasalazine
Type IIExtracellularType IIBSystemic circulation and other extracellular fluid compartmentsAcetylsalicylate, bacampicillin, bambuterol, chloramphenicol succinate, dipivefrin, fosphenytoin, lisdexamfetamine, pralidoxime
Type IIExtracellularType IICTherapeutic target tissues/cellsADEPTs, GDEPTs, VDEPTs


Subtypes

Type IA prodrugs include many antimicrobial and chemotherapy agents. Type IB agents rely on metabolic enzymes, especially in hepatic cells, to bioactivate the prodrugs intracellularly to active drugs. Type II prodrugs are bioactivated extracelluarly, either in the milieu of GI fluids, within the systemic circulation and/or other extracellular fluid compartments, or near therapeutic target tissues/cells, relying on common enzymes such as esterases and phosphatases or target directed enzymes. Importantly, prodrugs can belong to multiple subtypes. A Mixed-Type prodrug is one that is bioactivated at multiple sites, either in parallel or sequential steps. For example, a prodrug, which is bioactivated concurrently in both target cells and metabolic tissues, could be designated as a "Type IA/IB" prodrug. When a prodrug is bioactivated sequentially, for example initially in GI fluids then systemically within the target cells, it is designated as a "Type IIA-IA" prodrug. Many antibody- virus- and gene-directed enzyme prodrug therapies and proposed nanoparticle- or nanocarrier-linked drugs can understandably be Sequential Mixed-Type prodrugs. To differentiate these two Subtypes, the symbol dash " - " is used to designate and to indicate sequential steps of bioactivation, and is meant to distinguish from the symbol slash " / " used for the Parallel Mixed-Type prodrugs.