Novel cyanothiouracil and cyanothiocytosine derivatives as concentration-dependent selective inhibitors of U87MG glioblastomas: Adenosine receptor binding and potent PDE4 inhibition

Abstract

Thiouracil and thiocytosine are important heterocyclic pharmacophores having pharmacological diversity. Antitumor and antiviral activity is commonly associated with thiouracil and thiocytosine derivatives, which are well known fragments for adenosine receptor affinity with many associated pharmacological properties. In this respect, 33 novel compounds have been synthesized in two groups: 24 thiouracil derivatives (4a-x) and 9 thiocytosine derivatives (5a-i). Antitumor activity of all the compounds was determined in the U87 MG glioblastoma cell line. Compound 5e showed an anti-proliferative IC₅₀ of 1.56 μM, which is slightly higher activity than cisplatin (1.67 μM). The 11 most active compounds showed no signficant binding to adenosine A₁, A_2A or A_2B receptors at 1 μM. Brain tumors express high amounts of phosphodiesterases. Compounds were tested for PDE4 inhibition, and 5e and 5f showed the best potency (5e: 3.42 μM; 5f: 0.97 μM). Remakably, those compounds were also the most active against U87MG. However, the compounds lacked a cytotoxic effect on the HEK293 healthy cell line, which encourages further investigation.

Keywords: Adenosine; Antitumor; HEK293; MCF7; PDE; Phosphosdiesterase; Thiocytosine; Thiouracil; U87.

Figure 1.

Synthesized compounds and pharmacologically active molecules bearing the same fragments.

Figure 2.

¹H NMR interpretation summary of the compounds

Figure 3.

Effect of increasing concentrations of 5b–i on viability of U87MG cells.

Figure 4.

Logaritmic curves of active compounds on PDE assay

Figure 5.

Graphical representation of IC₅₀ values on all tested cell lines. An HEK293 bar is not given for 5d and 5e because the viability was more than 50%, even at 100 μM, and therefore IC₅₀ values could not be calculated.

Scheme 1.

Synthesis of the compounds 1a–c (i: aryl thioamide, 3-chloro-2,4-pentandione, EtOH, reflux 8 h, NaOAc; ii: product of i, CHCl₃,0°C, Br₂, r.t. 12 h, 30 min at 40°C

Scheme 2.

Synthesis of the compounds 2a–h and 3a–c (i: aldehyde, thiourea, K₂CO₃, ethyl cyanoacetate, EtOH, reflux 16h; ii: aldehyde, thiourea, K₂CO₃, malononitrile, EtOH, reflux 5h)

Scheme 3.

Synthesis of the compounds 4a–x and 5a–i (i: 2a–h, 1a–c, acetone, K₂CO₃, r.t. 12h, reflux 1 h, i: 3a–c, 1a–c, acetone, K₂CO₃, r.t. 8 h, reflux 1 h).