CHAPTER 1
DESIGN, SYNTHESIS, AND APPLICATIONS OF NUCLEOSIDE PHOSPHATE AND PHOSPHONATE PRODRUGS

JUNBIAO CHANG1,2 AND WENQUAN YU1

1 College of Chemistry, Zhengzhou University, Zhengzhou, China

2 School of Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, China

1.1. INTRODUCTION

For decades, numerous nucleoside analogs have been developed as drugs or drug candidates [1, 2] for the treatment of various cancers and infections by different viruses, such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes simplex virus (HSV), and human cytomegalovirus (HCMV). In cells, nucleoside analog molecules are converted to the corresponding phosphates by nucleoside or nucleotide kinases [3, 4] and then are incorporated into viral or tumor DNA/RNA where they serve as chain terminators. For example, the anti-HIV nucleoside drug azvudine (FNC) [4-6] is further phosphorylated in vivo to the active nucleoside triphosphate (FNC-TP, Figure 1.1), which inhibits the viral replication. However, the structural differences between the nucleoside analogs and the natural nucleosides may affect the phosphorylation processes, thus decreasing the pharmacological activity of the compounds [7, 8]. Due to their poor chemical stability and high polarity, the corresponding phosphates themselves cannot be used directly as drugs (Figure 1.2). To address these issues, diverse prodrugs of nucleoside (nucleotide) phosph(on)ate analogs have been designed and synthesized in drug discovery [3,9-14].

Figure 1.1 Formation in vivo of the active triphosphate (TP) of FNC.

Figure 1.2 Design and mechanism of action of nucleoside phosph(on)ate prodrugs.

Introduction of phosph(on)ate esters as prodrugs (Figure 1.2) has emerged as a very useful tool in the design and discovery of nucleoside and nucleotide analog drugs for the treatment of cancers and viral infectious diseases. The features of a prodrug may include (1) improved chemical stability; (2) increased lipophilicity for better bioavailability; (3) oral availability (the parent compound may often be administered only by injection); and (4) an improved therapeutic effect with reduced toxicity via targeted drug delivery. In recent years, considerable effort has been devoted to the design, synthesis, and biological evaluation of nucleoside phosph(on)ate prodrugs and more than twenty drug candidates have entered clinical development. Of these, several prodrugs have received FDA approval for clinical use. These include ADP, TDF, sofosbuvir, TAF, and remdesivir (Figure 1.3). In this chapter, four main classes of nucleoside phosph(on)ate prodrugs and prodrug candidates are discussed: carbonyloxymethyl diester prodrugs, alkoxyalkyl monoester prodrugs, cyclic 1-aryl-1,3-propanyl ester (HepDirect) prodrugs, and phosphoramidate/phosphonamidate prodrugs.

Figure 1.3 Nucleoside phosph(on)ate prodrugs in clinical use.

1.2. NUCLEOSIDE PHOSPH(ON)ATE PRODRUGS

1.2.1. Carbonyloxymethyl Diester Prodrugs

Two nucleotide prodrugs developed via this strategy, ADP (trade names: Preveon and Hepsera) and TDF (trade name: Viread), have been approved by FDA for the treatment of viral infectious diseases. Both the acyclic nucleoside phosphonate drugs have a carbonyloxymethyl diester moiety. Specifically, ADP contains a bis(pivaloyloxymethyl) (POM) substructure and TDF bears a bis(isopropyloxymethyl carbonate) group (POC). These carbonyloxymethyl (e.g., POM) and alkyloxycarbonyloxyalkyl (e.g., POC) diester units can increase the oral bioavailability and overall systemic exposure when compared to the parent nucleotide molecules. Activation of this carbonyloxymethyl ester prodrug involves the esterase-catalyzed cleavage of the first carbonate ester group followed by chemical degradation to form an unstable POM- or POC-monoester (2). The monoester intermediate then undergoes a second degradation cycle to afford the expected nucleoside monophosph(on)ate (1) (Figure 1.4). In addition to the above two marketed nucleotide prodrugs, besifovir (LB80380) is a bis(POM)-type prodrug candidate of an acyclic purine nucleotide analog and has entered clinical development for the treatment of HBV infection.

Figure 1.4 Activation of carbonyloxymethyl ester prodrugs.

1.2.1.1. Adefovir Dipivoxil

Adefovir (ADV), also known as 9-(2-phosphonomethoxyethyl)adenine (PMEA), is an acyclic nucleotide analog of adenosine monophosphate. ADV was first reported by De Clercq et al. [15] as having potent antiviral activity against HIV and other retroviruses [16, 17]. To increase the lipophilicity and intestinal permeability of the parent compound (ADV) for oral administration, various prodrugs of adefovir have been designed. However, the previously designed prodrugs with simple alkyl diester or amide groups failed, probably due to their inefficient degradation to adefovir in vivo [18], and the monoesters showed poor oral bioavailability because of the unmasked negative charge. Eventually, ADP (bis-POM PMEA, Figure 1.5) designed by masking the phosphonic acid moiety with two POM groups was identified and was found to have favorable lipophilicity and intestinal permeability. It can be conveniently metabolized to the parent nucleotide adefovir in vivo, leading to high oral bioavailability. ADP was initially developed for the treatment of HIV infections (AIDS) [19, 20], but failed due to its toxicity. Further development demonstrated that this ADV prodrug can significantly reduce the HBV viral load at a nontoxic dose [21, 22]. ADP (Preveon and Hepsera) received FDA approval for the treatment of chronic HBV infection via oral administration as a reverse transcriptase (RT) inhibitor.

Figure 1.5 Structure of adefovir dipivoxil.

1.2.1.2. Tenofovir Disoproxil Fumarate

Tenofovir, (R)-9-(2-phosphonylmethoxy-propyl)adenine (PMPA), was designed by incorporation of a methyl group into the side chain of adefovir. As a nucleotide analog reverse transcriptase inhibitor (nRTI), PMPA exhibits good inhibitory activity against HIV and other retroviruses [23]. However, like other nucleotide analogs, the parent nucleotide analog suffers from poor oral bioavailability. To improve the oral delivery and also avoid the side effects [24] from pivalic acid derived from ADP, new prodrugs were designed and evaluated. Among these, the bis(isopropyloxycarbonyloxymethyl) ester (bis-POC) prodrug (tenofovir disoproxil) displayed favorable solubility, stability, and improved oral bioavailability compared to the parent nucleotide and thus was selected for clinical development [25, 26]. TDF (Figure 1.6) has been approved by the FDA for the treatment of HIV/AIDS as well as chronic hepatitis B under the trade name Viread.

1.2.1.3. Besifovir

Besifovir (LB80380, Figure 1.7) is a bis(POM) phosphonate prodrug of a 2-aminopurine nucleotide analog and is in clinical trials for the treatment of HBV infection [27]. LB80380 displayed potent anti-HBV activity against wild-type and drug-resistant strains [28]. In the liver, the prodrug will be rapidly degraded to release the parent nucleotide, which is then oxidized at the C6-position of the purine moiety followed by phosphorylation to form the bioactive metabolites that serve as inhibitors of HBV DNA replication [27, 29].

Figure 1.6 Structure of tenofovir disoproxil fumarate (TDF).

Figure 1.7 Structure of besifovir (LB80380).

Figure 1.8 Structure of non-nucleoside prodrugs BMS-188494 and ER-27856.

Application of the carbonyloxymethyl ester prodrug strategy to non-nucleoside compounds led to the discovery of BMS-188494, a bis(POM)-substituted squalene synthase inhibitor (Figure 1.8), which is in clinical trials for the treatment of hypercholesterolemia [30]. Another squalene synthase inhibitor bearing three POM groups (ER-27856) (Figure 1.8) was demonstrated to have a cholesterol-lowering effect [31] and inhibitory activity on the growth of Trypanosoma cruzi [32].

1.2.2. Alkoxyalkyl Monoester Prodrugs

In most of the phosph(on)ate prodrugs, both oxygens of the phosph(on)ate groups are fully masked, but in this prodrug approach, only one of them is masked and the other remains as a free OH group. The concept underlying the alkoxyalkyl monoester prodrug design is to mimic the structural features of the natural lysophosphatidylcholine (LPC). Further structural optimization of the masking groups led to hexadecyloxypropyl (HDP) and octadecyloxyethyl (ODE) derivatives, two important prodrug moieties. These prodrugs are orally available, and they may...