Aiming to optimize macrocyclic lathyrane-type diterpenes as effective Pgp modulators, the phytochemical study of the methanolic extract of Euphorbia boetica aerial parts was carried out. Two new macrocyclic 6,17-epoxylathyrane-type diterpenes, named epoxyboetiranes A (1) and B (2), along with three known analogues (3-5) were isolated. Epoxyboetirane A (1), a triacetate isolated in large amounts, was hydrolyzed to give epoxylathyrol (6). In order to study the effect of the substitution pattern of the macrocyclic scaffold on MDR reversal, 6 was acylated with aroyl, phenylacetyl, cinnamoyl and alkanoyl chlorides/anhydrides, yielding eight new esters, epoxyboetiranes C-J (7-14). The ability of compounds 1-14 as P-glycoprotein (Pgp, ABCB1) modulators was evaluated through combination of transport and chemosensitivity assays, using L5178Y mouse T lymphoma cell line transfected with the human MDR1 gene. In the transport assay, excepting 1, 3 and 6, the compounds, at non-cytotoxic concentrations, displayed strong MDR reversing activity in a dose-dependent mode, exhibiting all the new acyl derivatives (7-14) a many fold increase in the activity when compared with 1. Apart from 11 and 12, all compounds exhibited remarkable synergistic effects in combination with doxorubicin. An ATPase assay, using membrane vesicles from mammalian cells overexpressing Pgp, was also performed with two representatives of the modulators (4 and 5). The results suggest that both compounds compete with substrates for the Pgp drug-binding sites.