What additional medication is most appropriate?

A 64-year-old man was reviewed in the diabetes clinic. He had a history of type 2 diabetes mellitus treated for 12 years. He had sustained a previous episode of acute kidney injury believed to be secondary to renal artery stenosis and exposure to an ACE inhibitor. He was being treated with metformin 500 mg three times daily and gliclazide 80 mg twice daily.

Investigations:

serum sodium143 mmol/L (137C144)

serum potassium4.4 mmol/L (3.5C4.9)

serum creatinine123 µmol/L (60C110)

estimated glomerular filtration rate (MDRD)51 mL/min/1.73 m2 (>60)

haemoglobin A1c75 mmol/mol (20C42)

He required a third drug that would not require dose adjustment if renal function were to decline in the future.

What additional medication is most appropriate?
A . alogliptin
B . linagliptin
C . saxagliptin
D . sitagliptin
E . vildagliptin