Key Inclusion Criteria:

Step 1:
1 HIV-1 infection documented by a rapid HIV test or any licensed ELISA test kit and confirmed by a repeat ELISA, Western blot, or plasma HIV-1 RNA at any time prior to study entry.

2 Triple-class experience or resistance at any time in the potential subject?s lifetime including the following:
? prior use of more than one NRTI for at least 3 months and prior use of at least one NNRTI for at least 3 months.
OR
? demonstration of at least one mutation from each of the NRTI and NNRTI class lists below on any historical resistance assay.
NRTI-associated mutations: M41L, A62V, K65R, D67N, 69 Insertion Complex, K70R, K70E, L74V, V75I, F77L, Y115F, F116Y, Q151M, M184V/I, L210W, T215Y/F, K219Q/E.
NNRTI-associated mutations: L100I, K103N, V106A/M, V108I, Y181C/I, Y188L/C/H, G190S/A, P225H, P236L.

3 Currently on a failing PI-containing regimen that includes at least 2 other ARVs besides the PI, with no regimen change for the 8 weeks prior to screening.
NOTE: An alteration in dose, dose frequency, or any within-class substitution(s) for intolerance is permitted.

4 Plasma HIV-1 RNA level ≥1000 copies/mL obtained within 30 days prior to entry on the current regimen using the ultrasensitive Roche Amplicor HIV-1 assay from a laboratory that possesses CLIA certification or its equivalent.
5 Negative result from a hepatitis B surface antigen test performed within 90 days prior to study entry.
6 The following laboratory values obtained within 30 days prior to Step 1 entry:
? Absolute neutrophil count (ANC)  750/mm3
? Hemoglobin  7.5 g/dL
? Platelet count  40,000/mm3
? Calculated creatinine clearance (CrCl)  60 mL/min, as estimated by the Cockcroft-Gault equation*
? AST (SGOT), ALT (SGPT), and alkaline phosphatase  5 x ULN
? Total bilirubin  2.5 x ULN

NOTE: If the potential subject is taking an indinavir- or atazanavir-containing regimen at the time of screening, total bilirubin ≤ 5 x ULN is acceptable.

* Calculation for the Cockcroft-Gault equation is available at https://www.fstrf.org/common/utilities/calculators/ccc.html

7 Females of reproductive potential (women who have not been post-menopausal for at least 24 consecutive months, i.e., who have had menses within the preceding 24 months, or women who have not undergone surgical sterilization, specifically hysterectomy, or bilateral oophorectomy or tubal ligation) will need a negative serum or urine pregnancy test within 48 hours prior to entry.
NOTE: Acceptable documentation of hysterectomy and bilateral oophorectomy, tubal ligation, tubal micro-inserts, and menopause is self-reported history.
8 All potential subjects must agree not to participate in the conception process (e.g., active attempt to become pregnant or to impregnate, sperm donation, in vitro fertilization), and if participating in sexual activity that could lead to pregnancy, the subject/ partner must use at least two reliable methods of contraception (condoms, with or without a spermicidal agent; a diaphragm or cervical cap with spermicide; an IUD; or hormone-based contraceptive) while receiving study treatment and for 6 weeks after stopping study treatment.
9 Men and women age 16 years.
10 Karnofsky score of ≥70% within 30 days prior to entry.
11 Willingness of subject to adhere to protocol requirements, especially with respect to randomized treatment assignment.
12 Ability to obtain prescription for nucleoside reverse transcriptase inhibitors and ritonavir and to have required prescriptions filled at time of entry to Step 2.
13 Ability and willingness of subject or legal guardian/representative to provide informed consent.

Step 2:
1 Receipt of successful phenotype/genotype resistance results from a plasma sample obtained at screening.

NOTE: Absence of results from a successful HIV-1 co-receptor tropism assay will not exclude a subject from Step 2, but will make him/her ineligible to receive maraviroc on study.

2 Identification of a study regimen and at least 2 NRTIs for use on study, selected from the options provided by the protocol regimen team.

NOTE: The selection must be reviewed and approved in writing by the site investigator. The investigator must also provide rationale for the selection.

3 Currently on the same failing PI-containing regimen for the entire duration of Step 1 that includes at least 2 other ARVs besides the PI.

4 The following laboratory values obtained within 14 days prior to Step 2 entry:

? ANC  750/mm3
? Hemoglobin  7.5 g/dL
? Platelet count  40,000/mm3
? CrCl  60 mL/min, as estimated by the Cockcroft-Gault equation*
? AST (SGOT), ALT (SGPT), and alkaline phosphatase  5 x ULN
? Total bilirubin  2.5 x ULN

NOTE: If the subject is taking an indinavir- or atazanavir-containing regimen at the time of screening, total bilirubin can be ≤ 5 x ULN.

*Calculation for the Cockcroft-Gault equation is available at https://www.fstrf.org/common/utilities/calculators/ccc.html

5 Females of reproductive potential (women who have not been post-menopausal for at least 24 consecutive months, i.e., who have had menses within the preceding 24 months, or women who have not undergone surgical sterilization, specifically hysterectomy, or bilateral oophorectomy or tubal ligation) will need a negative serum or urine pregnancy test within 48 hours prior to entering Step 2.
NOTE: Acceptable documentation of hysterectomy and oophorectomy, tubal ligation, tubal micro-inserts, and menopause is self-reported history.
6 All subjects must agree not to participate in the conception process (e.g., active attempt to become pregnant, in vitro fertilization), and if participating in sexual activity that could lead to pregnancy, the subject/partner must use at least two reliable methods of contraception (condoms, with or without a spermicidal agent, diaphragm or cervical cap with spermicide, an IUD; or a hormone-based contraceptive) while receiving study treatment and for 6 weeks after stopping study treatment.
7 Karnofsky score of ≥70% within 30 days prior to entry.