compounds which may be chemically or biologically modified as derivatives of the rapamycin nucleus while still retaining immunosuppressive properties accordingly the term u201c a rapamycin u201d includes esters ethers oximes hydrazones and hydroxylamines of rapamycin as well as rapamycins in which functional groups on the nucleus have been modified for example through reduction or oxidation the term u201c a rapamycin u201d also includes pharmaceutically acceptable salts of rapamycins which are capable of forming such salts either by virtue of containing an acidic or basic moiety as used herein pharmaceutically acceptable salts include but are not limited to hydrochloric hydrobromic hydroiodic hydrofluoric sulfuric citric maleic acetic lactic nicotinic succinic oxalic phosphoric malonic salicylic phenylacetic stearic pyridine ammonium piperazine diethylamine nicotinamide formic urea sodium potassium calcium magnesium zinc lithium cinnamic methylamino methanesulfonic picric tartaric triethylamino dimethylamino and tris hydroxymethyl aminomethane additional pharmaceutically acceptable salts are known to those skilled in the art in one embodiment the esters and ethers of rapamycin are of the hydroxyl groups at the 42 and or 31 positions of the rapamycin nucleus esters and ethers of a hydroxyl group at the 27 position following chemical reduction of the 27 ketone and that the oximes hydrazones and hydroxylamines are of a ketone at the 42 position following oxidation of the 42 hydroxyl group and of 27 ketone of the rapamycin nucleus in another embodiment 42 and or 31 esters and ethers of rapamycin are described in the following patents alkyl esters u s pat no 4 316 885 aminoalkyl esters u s pat no 4 650 803 fluorinated esters u s pat no 5 100 883 amide esters u s pat no 5 118 677 carbamate esters u s pat no 5 118 678 silyl ethers u s pat no 5 120 842 aminoesters u.