Designed heterodimerizing leucine zippers with a range of pIs and stabilities up to 10(-15) M

We have designed a heterodimerizing leucine zipper system to target a radio nuclide to prelocalized non-internalizing tumor-specific antibodies. The mo dular nature of the leucine zipper allows us to iteratively use design rule s to achieve specific homodimer and heterodimer affinities. We present circ ular-dichroism thermal denaturation measurements on four pairs of heterodim erizing leucine zippers. These peptides are 47 amino acids long and contain four or five pairs of electrostatically attractive g <----> e ' (i, i ' +5 ) interhelical heterodimeric interactions. The most stable heterodimer cons ists of an acidic leucine zipper and a basic leucine zipper that melt as ho modimers in the micro (T-m = 28 degreesC) or nanomolar (T-m = 40 degreesC) range, respectively, but heterodimerize with a T-m >90 degreesC, calculated to represent femtamolar affinities. Modifications to this pair of acidic a nd basic zippers, designed to destabilize homodimerization, resulted in pep tides that are unstructured monomers at 4 muM and 6 degreesC but that heter odimerize with a T-m = 74 degreesC or K-d(37) = 1.1 x 10(-11) M. A third he terodimerizing pair was designed to have a more neutral isoelectric focusin g point (pI) and formed a heterodimer with T-m = 73 degreesC. We can tailor this heterodimerizing system to achieve pharmacokinetics aimed at optimizi ng targeted killing of cancer cells.