This study uses a sequential dose-escalation design for single dose subcutaneous and intramuscular administrations in HIV-negative volunteers. The study investigates seven TMB-607 dose groups: Cohort 1: 200 mg subcutaneously (Arm A), 500 mg subcutaneously (Arm B) and 1,000 mg subcutaneously (Arm C); Cohort 2: 100 mg intramuscularly (Arm A), 400 mg intramuscularly (Arm B), 800 mg intramuscularly (Arm C) and 1,500 mg intramuscularly (Arm D). Five participants will be enrolled in each dose group, and will be randomly assigned in a 4:1 ratio to receive TMB-607 or placebo in a double-blinded fashion (four participants per group assigned to active TMB-607; one participant per group assigned to placebo). In each group, a single dose injection of TMB-607 or placebo will be administered at Day 0. The first three Day 0 injections administered in each dose group must be given at least 24 hours apart. Subjects will be enrolled sequentially so that a maximum of three subjects in each Cohort is dosed within any 24-hour period. All subjects will be monitored for 24 hours after study drug administration, with vital signs and ECGs checked hourly, and intensive pharmacokinetic samples collected during this period. Study drug will only be administered on Mondays to prevent weekend days from interfering with the daily assessments closest to the time of dosing. Participants will be followed for 10 weeks after receiving study drug.