phase II clinical trials to treat Squamous Cell skin carcinomas

Compound EL03p43, a PKz5b inhibitor, recently entered phase II clinical trials to treat squamous cell skin carcinomas. Initial results were extremely promising, with tumour masses decreasing by > 98% across all drug-treated patients.

However, after 38 weeks of treatment, in approximately 13% of patients, the tumours were observed to be again proliferating at a rate similar to that observed in pre-treatment.

Based on prior studies, which included co-crystallisation studies of EL03p43-wild-type PKz5b (a; key binding interactions shown below), it was suspected that resistance was most likely a result of a point mutation at Residue AA92 (colour red).

Based on genomic sequencing, recombinant a version of the PKz5bA92 mutant enzyme was constructed. However, the exact point and nature of the mutation could not be determined.

To gain insight into the exact amino acid mutation, the PKz5bA92 mutant enzyme was subjected to assays against EL03p43 analogues (i.e. compounds 1 – 8). At the same time, compounds 1 – 8 were assayed against whole cancer cells, human serum albumin (HSA; main protein of human blood plasma, high Ki values = lower binding affinity), and cytochrome P450 (CYP450; an important liver-metabolizing enzyme).

Using the results which are tabulated below, propose an explanation for each of the following questions.

Activity against the PKz5bA92X mutant

1. Based on the IC₅₀ values of the EL03p43 analogues (compound 1- 8) against the PKz5bA92 mutant enzyme, would you expect compound 9 to have a higher or lower activity relative to the lead compound?
2. Provide a brief explanation as to how you concluded your response to the above question (i.e. part a).
3. Based on your responses to parts a and b, of the 20 standard amino acids, propose which you suspect the Phe92 has been replaced within the PKz5bA92 mutant?
4. Propose the structure of a compound which you believe would have higher inhibitory activity against the PKz5bA92 mutant enzyme.

Therapeutic potential

1. Based on the EC50 values, Ki values against HSA, and CYP450 degradation rate against which analogue displays the most favourable therapeutic potential.

For the compound you suggested for part d (above), propose a structural modification to this analogue which could increase overall therapeutic potential.