The precise SIDDHARTA value of the energy shift and width of kaonic hydrogen has awaken a renewed interest for the meson-baryon interaction in the S=-1 sector. Our study has been carried out based on a chiral SU(3) Lagrangian up to next-to-leading order (NLO) and implementing unitarization in coupled channels, since the presence of Λ(1405) resonance makes not applicable a perturbative treatment of the Chiral Lagrangian in the energy region we are dealing with. The parameters of the model have been fitted to a large set of experimental K^- p scattering data in different two-body channels, to threshold branching ratios, and to the above cited data from SIDDHARTA. In contrast to other groups we take into consideration the KΞ channels which are very important to obtain more reliable values of the fitting parameters, in particular the NLO coefficients. We have shown that the K^- p →KΞ reactions are very sensitive to the NLO terms of the Lagrangian and also to the Born direct and cross diagrams. This fact is due to the null direct contribution of the Weimberg-Tomozawa (WT) term to the scattering amplitude of these particular reactions. In order to study the accuracy and stability of our fitting parameters, we include high-spin and high-mass resonances applying the Rarita-Schwinger method. On the other hand, the K^- p →ηΛ,ηΣ^0 reactions have associated pure isospin 0 (I=0) and isospin 1 (I=1) amplitude, respectively, and therefore, they constrain the relevance of each isospin component. In general, a good description in terms of the isospin decomposition is crucial to reproduce properly processes in which a single isospin component is filtered, such as for instance the K_L^0 p →K^+ Ξ^0 reaction which could be measured at the proposed secondary K_L^0 beam at Jlab and which is a pure I=1 process. In order to provide new isospin filtering scenarios from which we can extract additional information about the NLO coefficients, the weak decay of the Λ_b into J/ψ ηΛ and J/ψ KΞ states have been studied. In addition, motivated by the fact that the recent observation of the hidden charm pentaquark state P_c (4450) at LHCb via the Λ_b→J/ψ K^- p decay, we have performed an exploratory study about the possibility of finding a hidden-charm strange pentaquark in the decay of the Λ_b into J/ψ ηΛ from the invariant mass spectrum of J/ψ Λ pairs.