This is a Phase II, international multicenter, double-blind, placebo-controlled, crossover study to assess the efficacy of SR419 in PHN subjects.

The study will evaluate the efficacy and safety of SR419 in PHN patients. Each subject will participate in the study for up to approximately 14 weeks. This includes a screening period, drug treatment period, and post-treatment safety follow-up period. This is a crossover design study, in which subjects will be administered with SR419 and placebo according to their randomized sequence, that is, eligible subjects will be randomized in a 1:1 ratio to 1 of 2 double-blind treatment sequences: 30 mg of SR419 administered TID followed by placebo administered TID or placebo administered TID followed by 30 mg of SR419 administered TID.

Inclusion Criteria:

1. Adult male or female over 18 years old;
2. Having neuropathic pain of postherpetic neuralgia (PHN) that persists for >3 months after the herpes zoster rash is healed, with the pain area of a continuous area of affected rash.
3. DN4 score is ≥4 at Screening;
4. Average PI-NRS score of PHN-associated neuropathic pain over the last 24 hours at Screening is ≥4 and ≤9;
5. Female subjects must be non-pregnant and non-lactating;

Exclusion Criteria:

1. Other pains that cannot be clearly differentiated from PHN and may interfere with PHN assessment;
2. Circumstances that may affect pain assessment as determined by the investigator, such as skin disorders in the affected skin area that may affect sensation;
3. Active herpes zoster infection at screening;
4. Serious acute or chronic medical condition that, as assessed by the investigator, could increase the risks in subjects for participating in the trial or taking the study drug, or interfere with the study results;
5. Previous administration of other study drugs within 30 days or 5 half-lives before the study intervention used in this study (whichever is longer).