Two pentacyclic triterpenes isolated from Maprounea africana, 1β-hydroxymaprounic acid 3-p-hydroxybenzoate [3], 2α-hydroxymaprounic acid 2,3-bis-p-hydroxybenzoate [4] and their respective hydrolyzed products [5] and [6], have been found to demonstrate potent inhibitory activity against HIV-1 reverse transcriptase. Over the past decade, substantial progress has been made in defining strategies for the treatment of human immunodeficiency virus (HIV) infection, the cause of acquired immunodeficiency syndrome (AIDS). Because reverse transcriptase (RT) is required for early proviral DNA synthesis (1), inhibition of the RT-catalyzed polymerization of DNA from viral RNA inhibits virus replication. RTs may be viral-specific and are thus considered viable chemotherapeutic targets. Most potent HIV-RT inhibitors are nucleoside analogs that are converted to triphosphates by cellular enzymes and act as chain terminators (2,3). As examples, the only drugs approved for use in HIV-1 infection to date are azidothymidine (AZT), didanosine (dideoxyinosine or ddI), and zalcitabine (dideoxycytidine or ddC) (4). Although these compounds have been shown to benefit HIV-infected individuals, there are toxic sideeffects associated with their use (4), and complete inhibition of viral replication is rarely achieved (4-6). In addition, the emergence of nucleotide-resistant HIV strains may complicate long-term therapy (7). Most reverse-transcriptase inhibitors [such as antimonotungstate (8) and suramin (9)] also inhibit cellular DNA or RNA polymerases, and the nonselectivity of such agents can contribute to in vivo toxicity (10,11). Hence, the discovery and characterization of agents capable of specifically inhibiting HIV RT without mediating a toxic response remains a high priority. Natural products serve as one source of structurally novel chemicals that are worth investigating as specific inhibitors of HIV RT. Previous screening of various natural products for HIV-1 RT inhibitory activity showed stringent structural requirements (12), since few compounds demonstrated potent activity. Natural product RT inhibitors, such as benzophenanthridine (13) and protoberberine (14) alkaloids, flavonoids (15,16), a variety of other compounds with phenolic hydroxy groups (17,18), and certain antibiotics (19), were found to inhibit HIV-1 RT (as well as HIV-2 RT) with similar potency (12). We have also shown that O-methylpsychotrine is a selective inhibitor of HIV-1 RT (20) and even greater activity was observed with HIV-2 RT and this compound (21). In our continuing efforts in this area of research, an extract derived from the roots of Maprounea africana Muell.-Arg. (Euphorbiaceae) collected in Tanzania, yielded four pentacyclic triterpenes: maprounic acid [1], maprounic acid acetate [2], 1β-hydroxymaprounic 3-p-hydroxybenzoate [3], and 2β-hydroxymaprounic acid 2,3-bis-p-hydroxybenzoate [4]. We currently report the potential of these four compounds to inhibit RT, as well as 5 and 6, the hydrolyzed products of 3 and 4. Compounds 3-6 were found to inhibit HIV-1 RT with IC₅₀ values in the range of 3-5 μM (Table 1). Under these reaction conditions, the most potent inhibitory activity we have previously observed with plant secondary metabolites have been mediated by fagaronine (IC₅₀= 13 μM) (12) and O-methylpsychotrine (IC₅₀=14 μM) (20). Thus, we consider compounds 3-6 potent inhibitors of reverse transcriptase. Compound 1 was approximately fivefold less active against HIV-1 RT, relative to compounds 3-6, and the acetate of 1 (compound 2) was approximately 35-fold less active. All six compounds were less active toward HIV-2 RT, relative to HIV-1 RT (Table 1). Compounds 3 and 4 and their hydrolyzed products (compounds 5 and 6) all have similar HIV RT inhibitory potency. The mechanism of inhibition is currently under investigation. This report comprises the first example of pentacyclic triterpenes that mediate potent inhibition of HIV RT. From this limited series of six compounds, we can surmise that the greater activity demonstrated by compound 5, relative to compound 1, is due to the presence of two hydroxy groups. Furthermore, the ester groups of compounds 3 and 4 can be removed, without loss of activity, based on the fact that compounds 5 and 6 are equipotent as HIV-1 RT inhibitors. These data indicate that other types of structural modifications might be readily implemented without loss of activity. As a result of these features, compounds of this structural class are prime candidates for drug development.