A hexapeptide obtained from human casein by enzymatic digestion has been purified, sequenced and synthesized; its structure is: Val‐Glu‐Pro‐Ile‐Pro‐Tyr. In vitro this hexapeptide stimulates the phagocytosis of opsonized sheep red blood cells by murine peritoneal macrophages. Administered intravenously to adult mice, it enhances the resistance to infection with Klebsiella pneumoniae.

Oligodeoxynucleotides covalently linked to an acridine derivative were targeted to part of the 3'-terminal sequence which is common to the eight RNAs of type A influenza viruses. The cytopathic effect of the virus on MDCK cells in culture was strongly decreased by a heptanucleotide covalently attached to the acridine ring. Control experiments using other oligonucleotide sequences showed that the effect was specific for the complementary sequence of the 3'-terminal region of the viral RNAs. The RNA transcriptase reaction of a type A virus was also selectively inhibited in vitro by the heptanucleotide-acridine conjugate. A type B influenza virus was used as a control. The common sequence at the 3' end of its eight viral RNAs is different from that of type A viruses. Three mismatches were expected with the heptanucleotide which was fully complementary to type A viral RNAs. This heptanucleotide had no effect on the cytopathic effect of a type B influenza virus. These results demonstrate that viral RNAs are specific targets for the oligonucleotide-acridine conjugate that inhibits the cytopathic effect of type A influenza viruses.

An octathymidylate covalently linked via its 3'-end to an acridine derivative inhibited the cytopathic effect of Simian Virus SV40 on CV-1 cells in culture. Control experiments revealed that this effect was virus-specific and did not arise as a result of oligonucleotide degradation by nucleases. A photoactive probe was covalently attached to the 5'-end of the oligonucleotide-acridine conjugate. Upon UV-irradiation, photocrosslinking was shown to occur at the A. T-rich region within the viral origin of replication. A local triple helix can form at moderate salt concentrations with two octathymidylate-acridine conjugates bound to the octaadenylate sequence. Alternatively the octathymidylate-acridine conjugate can bind to the major groove of duplex DNA forming a local triple helix. Different mechanisms are discussed to explain the inhibition of viral DNA replication.

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