Exploring acetaminophen prodrugs and hybrids: a review

Abstract

This critical review highlights the advances in developing new molecules for treating pain syndrome, an important issue for human health. Acetaminophen (APAP, known as paracetamol) and nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used in clinical practice despite their adverse effects. Research is being conducted to develop innovative drugs with improved pharmaceutical properties to mitigate these effects. A more practical way to achieve that is to study well-known and time-tested drugs in their molecular combinations. Accordingly, the present work explores APAP and their combined chemical entities, i.e., prodrugs (soft drugs), codrugs (mutual prodrugs), and hybrids. Due to their molecular structure, APAP prodrugs or codrugs could be considered merged or conjugated hybrids; all these names are very fluid terms. This article proposed a structural classification of these entities to better analyze their advances. So, the following: carrier-linked O-modified APAP, -linked N-modified APAP derivatives (prodrugs), and direct- and spacer-N,O-linked APAP hybrids (codrugs) are the central parts of this review and are examined, especially ester and amide NSAID-APAP molecules. The C-linked APAP and nitric oxide (NO)-releasing APAP hybrids were also briefly discussed. Prime examples of APAP-based drugs such as propacetamol, benorylate, acetaminosalol, nitroparacetamol, and agent JNJ-10450232 weave well into this classification. The proposed classification is the first and original, giving a better understanding of the SAR studies for new pain relievers research and the design development for the analgesic APAP-(or NSAID)-based compounds.

Fig. 1. Graphical representation of APAP discovery and its main clinical pharmacological activities. APAP was synthesized in 1878 by Morse, and in 1887, von Mering used it clinically but discarded it because of the false assumption of its toxicity. In 1948, Brodie and Axelrod indicated the practical usefulness of the APAP (“APAP rediscovery”) that led to marketing in the 1950s in the United States.

Fig. 2. The predominant hypothesis on the mechanism of APAP action mode is the so-called “COX-associated Peroxidase Hypothesis”. The biosynthesis of PGs from ACA involves a key bifunctional enzyme containing two separate catalytic domains that are responsible for converting arachidonic acid to PGH₂: (i) a cyclooxygenase domain that engenders an unstable peroxide intermediate (PGG₂) that is blocked by non-selective and selective NSAIDs and (ii) a peroxidase (POX) domain containing a heme group that converts the unstable intermediate to PGH₂ where APAP inhibits the POX catalytic step by converting its heme group (Fe(iv)OPP˙⁺) to an inactive reduced state, – Fe(iii).

Fig. 3. Schematic representation of the predominant theories on the APAP metabolism in humans. Liver APAP metabolism. Main metabolic pathways of APAP in the liver after administration of therapeutic or toxic doses. Glucuronidation (non-toxic APAP–Glu formation) is the main pathway of acetaminophen metabolism, followed by sulfation (non-toxic APAP–SO₃H formation) and a minor contribution from the oxidation route (toxic NAPQI formation). Rapid conjugation of NAPQI to glutathione (GSH) allows detoxification of NAPQI through non-toxic glutathione or cysteine conjugates, which are eliminated in urine. CNS (brain) APAP metabolism. APAP deacetylation process to produce para-aminophenol (p-AP). Its conjugation with arachidonic acid by fatty acid amide hydrolase (FAAH) enzymes forms N-arachidinoyl-phenolamine (AM404), which exhibited TRPV1 agonism with an EC₅₀ value of 26 nm.

Fig. 4. Structures of APIs, which form fixed-dose combinations with APAP and non-opioid analgesics (NSAIDs) or opioid (narcotic) pain relievers, and pictorial representation of their combination: (A) drug cocktails combine more than one drug (API) simultaneously. It can be more effective than single drugs. Drug combinations may enhance the therapeutic effect of individual drugs, while drug–drug interactions are generally unfavorable. (B) The multiple-targeting double-fixed-dose drug combination approach with the participation of APAP and other APIs (API′).

Fig. 5. General classification of prodrugs using chemical criteria and a simplified illustration of the prodrug concept. Historically, Albert and Harper introduced the terms prodrugs and latentiated drugs to describe biologically active compounds that undergo biotransformation before revealing their pharmacological effects. Currently, according to their chemical structure, mechanism of activation, and modified functional groups, prodrugs are sorted into a bioprecursor group and carrier-linked prodrugs, also known as conventional prodrugs.

Fig. 6. Classification of molecular hybrids and their structural representation and the integration of their pharmacophores in a bioactive single hybrid molecule, i.e., API.

Fig. 7. Structures of the first synthetic analgesic drugs, acetanilide and phenacetin, resulted in precursor prodrugs of acetaminophen. Nevertheless, both were not originally designed as prodrugs, so their prodrug nature was determined in hindsight. Both are more toxic (renal toxicity) and less active (antipyretic and analgesic activities) than their metabolite, APAP, which is formed through aromatic hydroxylation and O-de-ethylating processes from the respective acetanilide and phenacetin prodrugs. They and their analogs comprise an APAP ring linked to single carrier moiety and are also known as bipartite prodrugs.

Fig. 8. Structures of the reported carrier-linked O-modified APAP prodrugs. The 4-AC-APAP (17) and 4-AOC-APAP (18) series were first examined to be administered orally, while the prodrugs 19–26 were developed and studied as topical analgesics. In general, such phenols, when administered orally, exhibit poor bioavailability. This major problem can be avoided with dermal delivery. Starting the 4-AOC-APAP prodrug 18 studies as topical analgesic agents, the AOC promoiety has been modified to enhance the topical delivery of APAP. Thus, numerous and different ACOM, AAC, DAAC, AOCOM, and NANAOCAM functional groups/fragments in the APAP core.

Fig. 9. Structures of the O-modified APAP prodrugs with amino acid (4-AAC-APAP prodrugs) or dipeptide carriers. Among them, parenteral APAP prodrug 27a (propacetamol) is an illustrative example. Nonspecific plasma esterases completely and rapidly hydrolyze propacetamol to APAP and diethylglycine in a 1 : 1 ratio. Thus, the IV administration of 1 g of propacetamol produces 0.5 g of APAP. Noteworthy that after the IV injection of propacetamol, APAP easily crosses the blood–brain barrier, guaranteeing a central analgesic effect. However, there is evidence that a single dose of IV propacetamol provides around four hours of effective analgesia for about 36% of patients with acute postoperative pain.

Fig. 10. Structures of APAP drugs based on polar, ionizable phosphate, sulfonate, or carboxylate moieties. Among them, sodium APAP phosphate 29 is in vivo hydrolyzable to APAP by alkaline phosphatase, abundant in the lumen of the small intestine. Prodrug 31 was also examined in vivo analgesic, antipyretic, and anti-inflammatory tests. Its anti-inflammatory action was better than APAP: its % inhibition of edema was 37.8% compared with that of APAP (31.7%).

Fig. 11. Structures of the reported carrier-linked N-modified APAP prodrugs with reduced hepatotoxicity. SCP-1 (37) was the first developed N-modified APAP lead. It contains the saccharin skeleton. Accidently discovered in 1878 by a Russian chemist Constantin Fahlberg, saccharin was the first commercially recognized as a sweet-tasting agent. N-Sulpharyl-APAP prodrugs 39 and 40 displayed similar analgesic properties (ED₅₀ = 45.2 μmol kg⁻¹ and 14.7 μmol kg⁻¹, respectively) to APAP (ED₅₀ = 68.6 μmol kg⁻¹) as well as antipyretic activity (ED₅₀ = 197.5 μmol kg⁻¹ and 176.6 μmol kg⁻¹, respectively), compared with APAP (ED₅₀ = 245.1 μmol kg⁻¹). JNJ-10450232 (NTM-006) agent (45) is a novel, promising analgesic and antipyretic APAP prodrug synthesized by Janssen Pharmaceutical Research & Development, LLC (JPRD, Spring House, PA). It was neurologically well-tolerated following single oral administration at up to the highest dose of 750 mg kg⁻¹ tested in Sprague-Dawley rats (modified Irwin Test).

Fig. 12. Proposed biotransformation pathways of non-hepatotoxic APAP derivatives 37–38, 43–44 and 45. The lack of hepatotoxicity was due to a decrease or lack of formation of the toxic quinone-imine type NAPQI. The elimination of JNJ-10450232 (45) and its major metabolites M₁–M₃ was mainly via urinary excretion. Derivatives 37–38 showed favorable cytochrome P450 isozyme profiles, and NAPQI generation by P450 isozymes was not detected in the serum of a mouse. Optical microscopy and confocal scanning light microscopy techniques indicated no considerable change in the hepatic tissues (liver of Wistar albino rats) after acute toxicity with treatments of 43–44, in contrast to the APAP treatment, in which distinct necrotic lesions were found that can be concluded that treatment of 43–44 is devoid of any toxic manifestation, i.e., NAPQI formation did not occur.

Fig. 13. Structures of merged hybrids containing two different APIs connected via a carboxylic ester function, so-called non-identical twin drugs, or mutual bipartite prodrugs, in which both of the drug molecules are directly linked to each other by a covalent bond. Benorylate (46) was first synthesized by Robertson in 1963. It is a white, odorless, tasteless, stable compound, very soluble in lipids, slightly soluble in ethanol, but practically insoluble in water. Acetaminosalol (47) did not hydrolyze in the gastric juice and was more slowly absorbed than acetylsalicylic acid APAP.

Fig. 14. Ester APAP hybrids 53–56 are potent inhibitors of FAAH and APAP hybrid 57 as promising multifunctional agents with high anti-nociception response associated with inflammatory pain. Pharmacological inactivation of FAAH produces analgesic, anti-inflammatory, anxiolytic, and antidepressant phenotypes without showing the undesirable side effects of direct cannabinoid receptor agonists. That indicates that FAAH may be a promising therapeutic target. All hybrids 53–56 contain the CF₃ group on the peripheral aryl ring. Thus, it is clear that the trifluoromethyl group is primarily responsible for activity in this series.

Fig. 15. Efficient drug delivery systems based on masked active benzoxazolone and oxazolidinone rings and structures of the respective linear carbamate–APAP hybrids 58 and 59–60, which release corresponding muscle relaxant drugs chlorzoxazone 58a, metaxalone 59a, and mephenoxalone 60a and APAP. Muscle relaxants are usually recommended for neck or back pain caused by muscle spasms. These medications help to reduce muscle spasms and tension.

Fig. 16. Structures of the amide NSAID–APAP codrugs (or linked N-modified APAP bipartite mutual prodrugs) 61–64 belonging to the merged hybrid group. They were designed and prepared with a view to combine the antipyretic activity component of APAP into the NSAID skeleton with their normal anti-inflammatory activity but without GIT ulceration. They were evaluated for their antipyretic activity in animal models (Male Sprague-Dawley rats). The pyrexia % values at a dose of 25 mg kg⁻¹ for 61–64 prodrugs were 87.4%, 59.5%, 55.8%, and 58.8%, while APAP showed a 63.8% reversal of body temperature. Their anti-inflammatory activity in the in vivo model using carrageenan-induced paw edema provided the following % inhibition values: 34.8% (61), 39.4% (62), 86.8% (63), and 68.9% (64).

Fig. 17. Structures of new conjugated hybrids containing APAP core: NSAID–AA–O–APAP hybrids 65–67, NSAID(Ibu)–AA–N–APAP hybrids 68, and Theoph–CH₂CH(OH)CH₂–O–APAP hybrids 69. They are also known as tripartite mutual prodrugs.

Fig. 18. Structures of the bis-direct-N,O-linked APAP codrugs 70–71 are attractive models for researching new APAP-based analgesics.

Fig. 19. Series of Bn–TA–APAP derivatives 72 and Ar–TA–APAP derivatives 73–74, interesting models for drug research. The 1,2,3-triazole ring plays the role of a linker unit.

Fig. 20. Series of the C-linked APAP compounds: chalcone–APAP (75), N-arylpyrazoline–APAP (76), and N-(pyridin-4-yl)pyrazoline–APAP (77) hybrids.

Fig. 21. Pictographic representation. (A) NO-donor hybrid APIs is a broad grouping that covers a range of established drugs that have been structurally modified to incorporate NO-containing molecules. The release of nitric oxide from NO-donor hybrid API must be balanced to provide sufficient activity within the concentration range of the parent API. (B) NO-donor hybrid APIs with a nitrate group, such as hybrid NO–NSAIDs, show interesting anti-inflammatory/analgesic properties and attractive effects in several cardiovascular conditions, especially in vascular injury, atherosclerosis, and anticancer therapy. These hybrids are examples of multitarget drugs, the single chemical entities that can modulate more than one target.

Fig. 22. Diverse NO-releasing APAP hybrids.