The key phosphorus moieties in drug design: antimicrobials and beyond

Owing to their diverse pharmacological activities and mechanisms of action, phosphorus-containing compounds have played a pivotal role in medicinal chemistry and drug design. In particular, various moieties have been employed in the design of small molecules, such as phosphonates, phosphates, phosphonopeptides, phosphonamidates, phosphoramidates, phosphinates, phosphine oxides and phosphonium compounds. The interest in the use of phosphorus in drug development is on the rise and extends to various therapeutic domains, beyond antimicrobials. Several phosphorus-containing drugs of high importance have been developed as therapeutic agents in the field of antimicrobials, antivirals, antitumor drugs, anti-osteoporotics, etc. [1,2].

The introduction of phosphorus atom into the molecule offers the ability to alter pharmacodynamics, pharmacokinetic, as well as related physicochemical properties, such as polarity and solubility. Phosphorus-containing moieties can serve as bioisosteres of various functional groups, including carbonyl, hydroxyl and carboxyl, often providing enhanced stability against metabolic degradation. In addition, they frequently resemble of biochemical entities, targeting various biological pathways and providing novel mechanisms of action. They are indeed moieties that deserve assiduous attention of medicinal chemists [1].

One of the commonly employed moieties in drug design that have made significant strides is the bisphosphonate group. Bisphosphonates, with general structure (HO)₂P(O)CR¹R²P(O)(OH)₂, have originally been developed for the treatment of osteoporosis and have shown promise for the treatment of bacterial infections in bone [3]. Bisphosphonates exhibit strong calcium-chelating properties, with strong affinity toward hydroxyapatite in bone – an effect that can be utilized in drug delivery methods. Recently, Ren et al. [4] presented evidence of methicillin-resistant Staphylococcus aureus (MRSA) eradication by bisphosphonate-conjugated sitafloxacin in osteomyelitis. Additionally, Guedeney et al. [5] reported on the synthesis of new azide, amino and maleimide alendronate analogs for conjugation strategies and discussed advantages and disadvantages of previous approaches. The demand for these linkers is expected to rise, as bisphosphonates are being developed both for targeting various bone infections and cancers, although there is no clinical derivative yet marketed. Prodrugging methods for highly charged bisphosphonates were thoroughly reviewed by Rudge et al. [6] in 2022 and may be useful for designing future strategies.

Quaternary phosphonium compounds (QPCs), species with a tetravalent phosphorus atom bearing a positive charge, are another group that has witnessed rapid development, especially in response to the COVID-19 pandemic. At present times, novel antibacterial disinfectants have been required to combat resistant pathogens and QPCs have emerged as a structurally novel class of compounds to do so. Strategies to develop biodegradable QPCs have been reported by Brayton et al. [7], addressing some of the pressing environmental and resistance issues of present times. Furthermore, the literature on the antimicrobial activities of various QPCs have been examined in a recent review by Ibrahim et al. [8], providing future directions for exploring the use of these compounds.

α-Aminophosphonates have attracted the attention of medicinal chemists due to their obvious similarity with α-amino acids. This structural feature has been implemented widely in the synthesis of novel antimicrobials, mainly employing Kabachnik-Fields or Pudovik reactions. With more than 15 new published articles in the past year, this area is still prolific and we herein introduce a few selected examples. Gadali et al. [9] prepared a series of 2-quinolone-derived α-aminophosphonates which displayed dual antiviral and antibacterial activity, presenting IC₅₀ values of 6.8–10.91 μM against yellow fever virus, Chikungunya virus and human rhinovirus and significant potencies against Enterococcus faecalis and S. aureus, with MIC values of 0.03 μmol/mL and 0.11 μmol/mL, respectively, for the most active derivatives. The compounds had moderate to good drug-like properties, likely not to cause any liver toxicity or carcinogenic effects. Kowalczyk et al. [10] extended their previous research [11,12] on antibacterial coumarin α-aminophosphonates prepared by environmentally-conscious enzyme-catalyzed Kabachnik-Fields reaction toward nosocomial pathogens. Five tested compounds exhibited higher antibacterial activity than that of commonly used antibiotics and structural features responsible for boosting the activity were discovered. In addition, two compounds revealed antimicrobial activity against all tested pathogens which included both Gram-positive and Gram-negative bacteria and all compounds active were discovered to be bactericidal agents. Synthesis using ionic liquid as a catalyst and solvent was employed in the preparation of novel antibacterial and anti-biofilm α-aminophosphonates by Neiber et al. [13]. MIC values of the most active compound were in the range of 3.13–6.25 μg/mL and a strong ability to eliminate mature bacterial biofilm and extracellular polymeric substances was also observed. The authors suggested a novel mechanism of action and reported on possible industrial applications of their discovery for developing anti-biofilm agents for clean water supply. Advances were also made in the field of metallo-β-lactamase inhibitors. Palica et al. [14] developed and studied novel α-aminophosphonates, where their P = O motif resembles the tetrahedral intermediate of β-lactam hydrolysis. Several non-cytotoxic compounds possessed IC₅₀ of <10 μM against NDM-1 and VIM-2 metallo-β-lactamases. The authors accompanied their synthetic and biological results by solution NMR spectroscopic and computational investigations, suggesting the modes of binding. Further studies are expected to yield α-aminophosphonates that are hoped to combat metallo-β-lactamase resistant bacteria.

As for clinically-marketed drugs, to the best of our knowledge, there have been only four phosphorus-containing antibacterial compounds approved. These include the antibiotics fosfomycin, clindamycin phosphate, tedizolid phosphate and ceftaroline fosamil. However, a large series of interesting cyclic boronates that additionally employ arylphosphonate group and target penicillin-binding proteins were patented by Venatorx Pharmaceuticals, Inc. [15]. It will be interesting to see if they progress into further clinical development.

Furthermore, there has been a rapid development in the field of antibacterial nanomaterials and polymers based on black phosphorus. Black phosphorus has a unique intrinsic antibacterial activity that has been connected to production of reactive oxygen species and has been studied solely in the last decade. Current advancements in black phosphorus nanomaterials with antibacterial properties, reviewed by Zhang et al. [16], hold promise for the development of effective antibacterial agents and materials that can combat bacterial infections and address the challenges posed by antibiotic resistance.

Regarding future outlook, recent advances in phosphorus chemistry have opened fruitful avenues for prospective drug design. Methodologies for mild conversions of carboxyl moiety to phosphonate have been developed, and phosphonylation of alkyl radicals has been added to the synthetic arsenal of medicinal chemists, independently by the groups of Aggarwal [17] and Li [18]. It is therefore expected that late-stage modifications to yield phosphonate analogues of current and newly-developed drugs will emerge in literature. This could prove to be useful especially in the field of metallo-β-lactamase inhibitors, where phosphorus-enabled interaction with zinc atoms present in β-lactamase enzymes could be expected, as demonstrated e.g., by nanomolar phosphonate inhibitors of various metallo-β-lactamases by Yan et al. [19]. Not only may this be relevant in the field of antibacterials, but also in the field of cytostatics, as various human metallo-β-lactamase fold enzymes are responsible for resistance to clinically important anticancer medicines [20]. Hand-in-hand then needs to proceed the development of prodrugging technologies for these purposes. While various methods have been reported for antivirals, such as the SATE, DTE, ProTide, CycloSal, HepDirect and other strategies, the prodrugging of phosphorus-based antimicrobials has not yet been widely employed. In addition, cytotoxicities of common prodrugs against normal cells need to be considered alongside their development [21], together with the chirality of novel phosphorus-containing derivatives [22]. The synthetic strategies toward P-stereogenic compounds were reviewed in 2020 by Lemouzy et al. [23] and in 2021 by Ye et al. [24] and these works provide a solid basis and direction for future design of alternative synthetic methodologies. Further, technologies to develop self-immolative prodrugs have been on the rise, providing access to novel drug-delivery systems and smart materials. These based on phosphorus are still scarce, however, the field is expected to expand as phosphorus has some valuable properties, such as a higher valency than carbon, due to which it can naturally accommodate alternative functionalities within one molecule, as reported by Šimon et al. [25]. Phosphorus moieties can improve the pharmacokinetic properties of therapeutic agents by modifying the solubility, stability and bioavailability, however, depending on the functional moiety, the effect may also be opposite in some derivatives. Especially, high polarity, low solubility and thus low bioavailability are the well-recognized problems of highly charged hydrolyzed phosph(on)ates, alongside with frequent synthetic and purification difficulties. Nevertheless, phosphorus moieties have become essential in drug design, providing distinctive chemical and biological properties that can be utilized to develop effective treatments across diverse medical fields.

Indeed, the potential of phosphorus chemistry is still expanding, with applications ranging from tackling antimicrobial resistance to providing new hope for antiviral and anticancer treatments. A variety of phosphorus-containing moieties have been employed in the search for novel antimicrobials and exciting methodologies for the incorporation of phosphorus moieties in drug design have been devised, both aiming to facilitate effective solutions to the most pressing health challenges that we currently face. As research progresses, phosphorus is hoped to play an important role in the development of the next generation of therapeutic agents.

Funding Statement

This work was supported by the Research Council of Finland under Project no. 348973, the University of Helsinki - Doctoral Program in Drug Research and the Gust. Komppa Fund of Alfred Kordelin Foundation.

Financial disclosure

This work was supported by the Research Council of Finland under Project no. 348973, the University of Helsinki - Doctoral Program in Drug Research and the Gust. Komppa Fund of Alfred Kordelin Foundation. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Writing disclosure

No writing assistance was utilized in the production of this manuscript.