Discovery, Chemistry, and Preclinical Development of Pritelivir, a Novel Treatment Option for Acyclovir-Resistant Herpes Simplex Virus Infections

Abstract

When the nucleoside analogue acyclovir was introduced in the early 1980s, it presented a game-changing treatment modality for herpes simplex virus infections. Since then, work has been ongoing to improve the weaknesses that have now been identified: a narrow time window for therapeutic success, resistance in immunocompromised patients, little influence on frequency of recurrences, relatively fast elimination, and poor bioavailability. The present Drug Annotation focuses on the helicase-primase inhibitor pritelivir currently in development for the treatment of acyclovir-resistant HSV infections and describes how a change of the molecular target (from viral DNA polymerase to the HSV helicase-primase complex) afforded improvement of the shortcomings of nucleoside analogs. Details are presented for the discovery process leading to the final drug candidate, the pivotal preclinical studies on mechanism of action and efficacy, and on how ongoing clinical research has been able to translate preclinical promises into clinical use.

Figure 1

Schematic overview of the discovery steps leading to first in vivo active analogues.

Figure 2

Various analogues of pritelivir (16) developed during the discovery process. Circles mark the sites of modifications, with pink ones leading to reduced activity (after data from Kleymann et al. and data on file).

Figure 3

Some representative inverse amide/urea analogues of pritelivir (16) during the discovery process (data from Kleymann et al. and on file).

Figure 4

Synthesis of pritelivir (16) and analogues: (a) disconnection approach to target molecules; (b) synthesis of thiazolyl sulfonamide reagents; (c) synthesis of diaryl acetic acids; (d) synthesis of some representative examples of pritelivir and analogues.⁻

Figure 5

Herpes simplex virus DNA replication as targeted by nucleoside triphosphates, for example, acyclovir, and helicase–primase inhibitors like pritelivir. The heterotrimeric helicase–primase complex is formed by UL5 (the helicase subunit, which unwinds and separates the duplex DNA to form the replication fork, exposing the two individual DNA strands), UL52 (the primase, which synthesizes short RNA primers as substrates for DNA polymerase), and UL8 (noncatalytic subunit, essential for coordination between UL5 and UL52). Single strand DNA is protected from spontaneous realignment by ICP8, also known as single-stranded DNA-binding protein. The heterodimeric DNA polymerase is composed of UL30 (catalytic subunit) and UL42 (processivity subunit). Arrows denote the direction in which the DNA polymerase complements the single strand templates. This enzyme strictly operates in the 5′–3′ direction so that one strand is processed continuously (toward the fork) and therefore faster (“leading strand”) and the other strand discontinuously (“lagging strand”). The latter process starts at multiple primed sites to produce short 5′–3′ segments, the so-called Okazaki fragments, which are linked subsequently. Detailed descriptions of these processes and proteins have been provided previously.^,

Figure 6

Interactions between pritelivir and amino acid residues of HSV-1 UL5 (helicase) visualized as pharmacophore features derived by LigandScout (version 4.4.7, Inte:Ligand GmbH, Vienna, Austria): hydrophobic interactions (yellow spheres) with Phe351 and Phe375, H-bond donor (green arrow) with His368, H-bond acceptor with Gln370 (red arrow), and π–cation interaction with Arg874 and Lys356 (blue ring with arrow pointing toward positively charged amino acids). Amino acids highlighted with a blue box represent those pritelivir resistant mutations in the UL5 gene protein in relation to the binding of pritelivir. The interaction features were obtained from the homology model built by Biswas and colleagues.

Figure 7

Murine lethal challenge assays comparing developmental compounds of the thiazole type (left panel) and existing HSV infection treatment modalities (right panel). Six hours after intranasal infection with HSV-1 or HSV-2, mice (n = 10 per group) were orally administered test items for 5 consecutive days, three times a day. Three-week survival rates were recorded. ED₅₀ is the dose at which 50% of the infected animals survived (after data from refs (11) and (26)).

Figure 8

Potency of pritelivir therapy in murine lethal challenge studies after delayed treatment onset (n = 15 per group). Acyclovir (ACV) was used as a reference compound in ACV-sensitive and -resistant HSV strains. Drugs were orally administered (twice daily), starting 72 h after intranasal infection with HSV-1 (left panel) or HSV-2 (right panel). Three-week survival rates were recorded (after data from ref (30)) ■ = ACV sensitive HSV, treatment ACV; □ = ACV resistant HSV, treatment ACV; ● =ACV sensitive HSV, treatment pritelivir; ○ = ACV resistant HSV, treatment pritelivir.

Figure 9

Effect of early (days 0–4 postinfection) and late treatment (starting on day 4 postinfection for 10 days) in a guinea pig model of genital herpes disease. Mean values ± SD (n = 10 per group) of cumulative lesion scores on day 12 are shown, i.e., on day 8 post-treatment with early and on day 7 of treatment with late treatment. Co, Control; Val, valacyclovir (150 mg/kg, b.i.d.); Prit, pritelivir (20 mg/kg, b.i.d.) (after data from ref (31)).

Figure 10

Nonenzymatic hydrolysis of pritelivir in humans.

Figure 11

pH dependent dissolution of pritelivir mesylate monohydrate 100 mg film-coated tablets (1000 mL, 37 ± 0.5 °C, 50 rpm, USP apparatus 2 (paddle)).

Figure 12

Dose-dependent decrease of virus shedding and lesion frequency in patients with recurrent genital herpes under treatment with pritelivir over 28 days (after data from ref (28)).