Cytotoxic platinum(II) alkynyl complexes of the type [(COD)Pt(Me)(C≡CR)] and [(COD)Pt(C≡CR)2] were evaluated for their cellular uptake, thioredoxin reductase (TrxR) inhibition and toxicity against zebrafish embryos. Quantification of the cellular uptake confirmed lower accumulation levels for the mono-alkynyl derivatives [(COD)Pt(Me)(C≡CR)] in comparison to the bis-alkynyl complexes [(COD)Pt(C≡CR)2]. Importantly, the complexes were efficient inhibitors of TrxR in contrast to cisplatin, which was not active. Toxicity studies in zebrafish showed lower activity if compared with the cytotoxicity against tumor cells indicating a suitable therapeutic index for the complexes.