The cure of cancer represents an ultimate challenge for scientists from different fields.

Cancer complexity and diversity hamper the discovery of a broadly applicable treatment,

and consequently, cancer represents the second cause of premature death worldwide.

Despite the continuous approval of new drugs, the cancer burden keeps increasing due to

different factors and leaves a vast number of patients helpless. Conventional

chemotherapy still represents the backbone of cancer medical care. However, these agents

are not able to selectively accumulate at the disease site which limits their efficacy and

cause severe side effects. In the last years, targeted therapy has appeared as an innovative

approach to overcome the drawbacks shown by traditional chemotherapeutics. In this

approach, a cytotoxic agent is directed to the tumor site through the covalent conjugation

to homing devices (e.g. antibodies, small molecules).

Cryptophycins are cyclic depsipeptides with natural origin that present high cytotoxicity

against several cell lines. Although cryptophycins cannot be used as stand-alone agent

due to their side effects, they hold great potential as cytotoxic agent for tumor targeted

therapy. Therefore, the discovery of new cryptophycins that can be conjugated to homing

devices and their vectorization could be translated in a significant therapeutic activity.

In the first part (chapter 3), the discovery of new cryptophycin analogues that retain the

high cytotoxicity of the parent compound and present a functional group that can be used

for conjugation to a delivery vehicle was described. Moreover, the usage of molecular

dynamics to predict the biological activity of new analogues was explored.

The second part (chapters 4 and 5), describes the usage of cryptophycin-55 glycinate as

payload in small molecule-drug conjugates (SMDCs). In chapter 4, the payload was

conjugated to a ligand capable to target the carbonic anhydrase IX, a transmembrane

enzyme that is widely overexpressed in tumors. The cytotoxic activity of the resulting

conjugate was studied in vitro, and the therapeutic activity was investigated in mice. In

chapter 5, the payload was further explored by coupling it to a cyclic peptide targeting

the somatostatin receptor 2, a marker which is commonly overexpressed in

neuroendocrine tumors. The cytotoxicity of the conjugates was evaluated in a cell-based

assay. In this case, further investigations in their targeting properties and stability were

performed. Finally, the antitumor activity of the lead compound was investigated in vivo.