The purpose of this work is to compute the autodetachment rates of the nsnp2 4P1/2 levels for Be (2s2p2), Mg (3s3p2) and Ca- (4s4p2). Fano's configuration interaction model is implemented to compute the interaction between the discrete and continuum spaces of the wavefunction, which are determined using the multiconfiguration Hartree-Fock technique. Analysing the decay mechanism of the 4P1/2 components, the authors find large cancellation effects in the transition matrix for the interaction between the discrete perturbers and the ns2ks2S1/2 continuum channels, which makes the calculations very sensitive to computational details. They predict relatively long-lived metastable states with lifetimes in the range 10-5-10-7 s, which, however, only for Be- remain in a qualitative agreement with the experiment.