Safety and Antiviral Study of ACH-126, 443 (Beta-L-Fd4C) in the Treatment of Adults With HIV Infection and Modestly Detectable Viral Load.

Inclusion Criteria: * Adults ≥18 years of age * Receiving a stable triple combination antiretroviral regimen including 3TC, one other NRTI and either an NNRTI or a protease inhibitor for at least 4 months (16 weeks) * Demonstration of initial viral suppression and subsequent rebound to be defined as an initial virological drop of at least 0.5 Logs on a 3TC-containing regimen * Plasma HIV RNA level \> 1000 and \< 30,000 copies/mL on two occasions * Genotypically documented M184V variant of HIV RT * Clinically stable HIV status with no AIDS-defining events * CD4 \> 200 cells/mm3 * Basic hematologic and chemistry parameters within acceptable limits (defined in protocol) * All women of child bearing potential must have a negative serum or urine pregnancy test (minimum sensitivity of 25 IU/L of b-HCG) within 72 hours prior to the start of study medication * No active opportunistic infection requiring treatment * Subject must be able to provide written informed consent * Baseline laboratory values measured within 28 days of initiating study drug as follows: * HGB≥9.0g/dl or HCT≥27% (in the absence of blood transfusions or erythropoietin treatment in the preceding two weeks * Absolute neutrophil count≥1000 cells/mm(\^3) (in the absence of on-going G-CSF therapy * Platelet count ≥75,000/mm(\^3) * AST \<7.0 times the upper limit of normal * ALT ,7.0 times the upper limit of normal * Serum creatinine \<1.1 times the upper limit of normal Exclusion Criteria * Evidence of active HBV infection as demonstrated by HBsAg positivity * Hepatitis C co-infection * Concurrent systemic antiviral treatment * Previous therapy with agents with significant systemic myelosuppressive or cytotoxic potential within 3 months of study start or the expected need for such therapy at study start. * Alcohol abuse * Pregnancy or breast-feeding * Inability to tolerate oral medication * AST \> 7.0 times the upper limit of normal * ALT \> 7.0 times the upper limit of normal * Any clinical condition or prior therapy that, in the Investigators opinion, would make the subject unsuitable for the study or unable to comply with the dosing requirements. * Use of any other drug or substance with anti-HBV activity