Rapid viral expansion and short drug half-life explain the incomplete effectiveness of current herpes simplex virus 2-directed antiviral agents

Abstract

The nucleoside analogues acyclovir (ACV) and famciclovir (FCV) reduce the frequency and severity of herpes simplex virus 2 (HSV-2) genital shedding, yet despite their high potency in vitro and a lack of induced drug resistance, frequent episodes of breakthrough mucosal shedding occur. We tested a published stochastic, spatial mathematical model of HSV-2 replication and spread, in concert with pharmacokinetic and pharmacodynamic equations, against virologic data from clinical trials of twice-daily acyclovir and famciclovir suppression. The model reproduced the key features of clinical trial data, including genital shedding episode rate, expansion and decay dynamics, and heterogeneous peak viral production and duration. In simulations, these agents shortened episode duration by limiting the extent of viral production by 1 to 2 log units and limiting the formation of secondary ulcers by ∼50%. However, drug concentrations were noninhibitory during 42% of the dosing cycle. Even if drug concentrations were high at episode initiation, prolonged episodes often ensued due to drug decay over ensuing hours and subsequent rebound of rapidly replicating HSV-2. The local CD8(+) T-cell density was more predictive of episode viral production (R(2) = 0.42) and duration (R(2) = 0.21) than the drug concentration at episode onset (R(2) = 0.14 and 0.05, respectively), though the model projected that an agent with an equivalent potency but a two times longer half-life would decrease shedding by 80% compared to that of standard twice-daily regimens. Therefore, long half-life is a key characteristic of any agent that might fully suppress HSV-2 reactivations.

Fig 1

Mathematical model. (A) Each shedding episode from the clinical trials was classified according to duration; first, peak, and last HSV DNA genomic copy value; and initiation-to-peak and peak-to-termination slopes (adapted from reference with permission of the publisher). (B) The stochastic model (adapted from reference 8) attempts to reproduce the heterogeneity of these episode features with 30 to 60 days of sampling, as in the trials. The model describes initial infection of susceptible keratinocytes (S) by neuron-derived virus (V_neu) with propagation of infection within a single ulcer by cell-associated viruses (V_i) from infected cells (I). CD8⁺ T cells (E) expand at rate q, kill infected cells at rate f · E, and then decay at rate d. Upon cell death, V_i converts to cell-free virus (V_e), which can initiate new ulcers in adjacent regions. Antiviral therapy is assumed to impact viral production (red arrows) from keratinocytes (p) and neurons (F) but not cell-associated or cell-free viral infectivity (b_i and b_e, respectively) or free-virus decay rate (c).

Fig 2

The mathematical model reproduces episode heterogeneity on and off antiviral therapy. Solid colored bars, results for 8,142 genital swabs (A) and 262 shedding episodes (B to G) for 156 study participants off therapy; solid black bars, results for 7,509 genital swab specimens (A) and 115 shedding episodes (B to G) for the same study participants on therapy. The modeled trial simulation, represented with colored trend lines (no therapy) and black trend lines (therapy) in panels A to E and striped colored bars (no therapy) and striped black bars (therapy) in panels F and G, continued until 543 and 264 episodes off and on therapy, respectively, were generated; model sampling occurred every 24 h, as in the clinical protocol. The model output reproduced the quantitative shedding frequency (A), as well as episode duration (B), peak HSV DNA copy number (C), first HSV DNA copy number (D), last HSV DNA copy number (E), and rate (F) and median (G) initiation-to-peak and peak-to-termination slopes on and off therapy. (B) Two separate analyses of the trial data in which episodes of unknown duration were censored or uncensored were performed.

Fig 3

Time since dosing has a minor effect on episode duration and peak viral load. The intensity of red shading indicates the drug level, and white spaces indicate drug concentrations less that the EC₅₀. C_max occurs 2 h after dosing. Light blue bars are mean values within 2-h intervals. (A and B) Data from 413 simulated episodes on twice-daily drug treatment (63 to 75 episodes per interval). (A) Peak viral production in the initial episode ulcer; (B) episode duration as a function of time since dosing. (C and D) Data from 540 simulated episodes on once-daily drug treatment (39 to 51 episodes per interval). (C) Peak viral load in the initial episode ulcer; (D) episode duration versus time since dosing.

Fig 4

The local CD8⁺ T-cell density at episode onset has a profound effect on local viral production and episode duration in model simulations. Data are from 413 episodes on twice-daily drug treatment, 540 episodes on once-daily drug treatment, and 777 episodes off drug. (A and B) CD8⁺ T-cell density at episode onset versus peak viral production in initial episode region; (C and D) CD8⁺ T-cell density at episode onset versus duration per episode.

Fig 5

A short antiviral half-life allows HSV breakthrough shedding in model simulations. (A and B) Log HSV DNA genomic copies and ACV concentration/EC₅₀ (ACV/EC₅₀) as a function of time; peaks in drug concentration lead to deceleration of viral expansion and frequent reversion to a viral clearance phase; virus reexpanded during trough levels. Episodes for which the results are depicted in panels A and B spread to 7 and 18 microanatomic regions, respectively. (C) Slope of viral expansion for the episode for which the results are depicted in B as a function of time; (inset) relationship of slope to drug level over a 24-h period. (D) Phase diagram displaying the slope of viral expansion in panel C as a function of ACV/EC₅₀. Peak drug levels lead to a decrease in the viral expansion slope, often, but not always, to below zero (viral contraction). The drug has its greatest relative impact on slope when the viral expansion slope is at its highest and has little effect when virus is already decaying.

Fig 6

Antiviral therapy decreases average episode duration by limiting regional peak viral production and decreasing the probability of secondary ulcer formation. (A) Empirical data from 115 episodes on drug and 262 episodes off drug; (B and C) simulated data from 413 episodes on drug and 777 episodes off drug; (A and B) episode duration as a function of peak HSV DNA copy number of the initiating ulcer during an episode: the linear relationship between these two outcomes disappears at peaks of >∼10⁶ HSV DNA copies due to formation of secondary ulcers, which prolong episodes and lead to nonmonotonic viral clearance; high-copy-number shedding and prolonged episodes are more common without drug. (C) Percentage of simulated episodes with secondary ulcers: 19% of episodes on drug versus 44% of episodes off drug have secondary ulcers.