A 46-year-old male presents with thirst, polydipsia, and weight loss for 3 months, followed by sudden coma for 2 days. Blood glucose: 41 mmol/L, serum sodium (Na): 132 mmol/L, serum potassium (K): 4.0 mmol/L, blood urea nitrogen (BUN): 9.8 mmol/L, CO<sub>2</sub> combining power: 18.3 mmol/L, strongly positive urine glucose and ketones. What is the first-line treatment?