TAMOXIFEN AND ESTROGEN LOWER CIRCULATING LIPOPROTEIN(A) CONCENTRATIONS IN HEALTHY POSTMENOPAUSAL WOMEN

Data in the literature suggest that circulating levels of lipoprotein( a) [Lp(a)] and insulinlike growth factor I (IGF-I) respond similarly t o therapy with growth hormone, estrogen, or tamoxifen. To more clearly document these relations, we designed a randomized, double-blind, pla cebo-controlled study of the effects of tamoxifen and continuous estro gen on circulating levels of Lp(a), IGF-I, and IGF binding protein 3 ( IGFBP-3) in healthy postmenopausal women. Both estrogen and tamoxifen decreased serum levels of IGF-I to 30% below baseline during the 3 mon ths of treatment, while IGFBP-3 levels were unchanged. Plasma Lp(a) le vels decreased to 24% below baseline after 1 month of treatment with e ither estrogen or tamoxifen (P < .05 for estrogen only); after 3 month s Lp(a) decreased to 34% below baseline with tamoxifen therapy (P < .0 5) but returned to only 16% below baseline with estrogen. The correlat ion between Lp(a) and IGF-I was highly significant (P < .0001). We con clude that (1) tamoxifen lowers plasma Lp(a) levels in healthy postmen opausal women, (2) the suppressive effects of tamoxifen and estrogen o n circulating Lp(a) concentration diverge after the first month of the rapy, and (3) circulating levels of Lp(a) and IGF-I are strongly corre lated with each other, an indication that they may share regulatory in fluences.