Amphiphilic porphyrins are widely used photosensitisers (PSs) in photodynamic therapy (PDT), an antitumor treatment in which the PS is combined with light irradiation of appropriate wavelength and oxygen in the vicinity within the treated lesions to induce the cytotoxic effect and consequently tumour destruction. In addition to their beneficial properties as antitumour agents, they are also used to treat viral, bacterial or fungal infections. The hydrophobic parts of the molecule facilitate the passage through the membrane bilayer, while the hydrophilic parts enable water solubility and thus the possibility of easier administration of the PS. Although the amphiphilic porphyrins have shown great efficacy as PSs, the hydrophilic-lipophilic balance, which utilises all the advantageous properties of this type of PSs and avoids the disadvantages such as aggregation or low cellular uptake, is somewhat less studied. The main goal of this doctoral thesis was to study the hydrophilic-lipophilic balance on different groups of asymmetric A3B pyridylporphyrins and its content is divided into four parts. In the first part, N-methylated (pyridinium-3-yl) porphyrins and Zn(II) analogues with alkyl chains of different lengths (7-17 C-atoms) were compared. The Zn(II)-porphyrin with a 13 C-atom alkyl chain showed the highest cytotoxicity and selectivity towards the melanoma cell line when irradiated with orange light. The second part involved the synthesis of amphiphilic N-oxidised pyrid-3-ylporphyrins, where porphyrins with a 13 and 17 C-atom chain showed to be promising against melanoma cell line, diminishing the melanin obstruction. In the last part, amphiphilic (pyridinium-3-yl)porphyrins were chelated with 68Ga(III) and natGa(III) to obtain the PET/PDT agents, and their distribution was analysed in PET-CT scan. It was found that the hydrophilic-lipophilic balance not only affects cytotoxicity and selectivity in vitro, but also exhibits differential biodistribution in vivo. These findings on A3B pyridylporphyrins will give us a deeper insight into the importance of the amphiphilicity of the molecule and lead us to the further development of an “ideal PS” for use in PDT.