The present disclosure provides lentiviral vectors comprising codon optimized Factor VIII sequences, and methods of using such lentiviral vectors. The liver-targeted lentiviral vectors disclosed herein can be used for gene therapy, wherein the lentiviral gene delivery enables stable integration of the transgene expression cassette into the genome of targeted cells (e.g., hepatocytes) of pediatric (e.g., neonatal) or adult subjects, achieving an improvement in FVIII expression (for example, a 100-fold improvement) at low lentiviral vector doses (e.g., 5x1010 or lower, such as 1.5x109 or lower, or 1x108 TU/kg or lower). The present disclosure also provides methods of treating bleeding disorders such as hemophilia (e.g., hemophilia A) comprising administering to a subject in need thereof a liver-targeted lentiviral vector comprising a codon optimized Factor VIII nucleic acid sequence at low dosages (1x108 TU/kg or lower to 1.5x1010 TU/kg).
