In vitro effects of doxycycline and tetracycline on mitogen stimulated lymphocyte growth.

Doxycycline and tetracycline were tested for inhibitory effect on PHA stimulated growth (measured by [3H]-TdR uptake) of peripheral blood mononuclear cells from 26 normal subjects. Doxycycline reduced DNA synthesis appreciably at concentrations within and just above the therapeutic range of blood levels: tetracycline did not inhibit DNA synthesis of 3 day cultures even at concentrations five-10 times greater than the therapeutic blood level. Detailed studies on the action of doxycycline by volume spectroscopy and RNA flow cytofluorimetry indicated that this drug did not influence the recruitment of cells into the first G1-phase. Cell counts, DNA flow cytofluorimetry and autoradiography after pulsed exposure to [3H]-TdR showed a reduction in the numbers of growing cells in 2 and 3 day cultures in the presence of doxycycline compared with those containing tetracycline at the same concentration.