Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
Insect Control: Biological and Synthetic Agents - Index of
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A large synthetic effort has gone into the modification<br />
<strong>of</strong> the forosamine <strong>and</strong> rhamnose moieties, as<br />
well as the core tetracycle (Crouse <strong>and</strong> Sparks, 1998;<br />
Crouse et al., 2001; Anzeveno <strong>and</strong> Green, 2002; Kirst<br />
et al., 2002b) (see Section 6.7). However, other<br />
approaches to macrolide modification are now possible.<br />
Sequencing the genes involved in spinosyn biosynthesis<br />
(Waldron et al., 2000, 2001; Madduri et al.,<br />
2001) has allowed the production <strong>of</strong> novel spinosyns<br />
through biotransformation <strong>and</strong> modification <strong>of</strong> the<br />
biosynthetic pathways. For example, a genetically<br />
engineered strain <strong>of</strong> Saccharopolyspora erythraea<br />
expressing the spnP glycosyltransferase gene was<br />
used to produce biotransformed spinosyns, wherein<br />
the b-d-forosamine moiety was replaced by a-lmycarose<br />
at the C17 position (Gaisser et al., 2002).<br />
Alternatively, modifications to the basic tetracycle<br />
have been obtained via loading module swaps in the<br />
PKS, leading to a variety <strong>of</strong> novel substitutions at<br />
C21 <strong>and</strong> other positions (Burns et al., 2003; Martin,<br />
2003). As one example, the ethyl group at C21 has<br />
been replaced with an n-propyl group (Martin,<br />
2003), resulting in a unique spinosyn as active as<br />
spinosyn A (Tables 1 <strong>and</strong> 3).<br />
6.2.4. Physical Properties <strong>of</strong> the Spinosyns<br />
The spinosyns are high molecular weight (approximately<br />
732 Da), relatively nonvolatile molecules<br />
(Table 4). As with the biological activity, small<br />
changes in structure can have far larger effects<br />
than might otherwise be expected. Although spinosyns<br />
A <strong>and</strong> D only differ by the presence <strong>of</strong> a methyl<br />
group at C6 (Table 1), this has a surprising effect on<br />
the physical properties, especially solvent solubilities.<br />
For example, spinosyn A is soluble in water at<br />
about 8.9 mg 100 ml 1 , while the water solubility <strong>of</strong><br />
spinosyn D is only 0.05 mg 100 ml 1 . This pattern<br />
holds true for other solvents as well (Table 4).<br />
6.3. Pharmacokinetics <strong>of</strong> Spinosad<br />
6.3.1. Metabolism <strong>of</strong> the Spinosyns<br />
6: The Spinosyns: Chemistry, Biochemistry, Mode <strong>of</strong> Action, <strong>and</strong> Resistance 213<br />
6.3.1.1. Mammalian <strong>and</strong> avian spinosyn<br />
metabolism Studies <strong>of</strong> 14 C-spinosyns A <strong>and</strong> D<br />
metabolism in rats identified fecal excretion as the<br />
major route <strong>of</strong> elimination for both spinosyns A <strong>and</strong><br />
D. In the fecal material, between 52% (males) <strong>and</strong><br />
28% (females) <strong>of</strong> the administered dose <strong>of</strong> spinosyn<br />
A was excreted in the feces as parent or metabolites<br />
within 0–24 h post-dosing. Of this material<br />
only 6.4% (males) <strong>and</strong> 5.4% (females) was spinosyn<br />
A (Domoradzki et al., 1996). For spinosyn D,<br />
between 67.7% (males) <strong>and</strong> 73.3% (females) <strong>of</strong><br />
the applied dose was present in the feces, <strong>of</strong> which<br />
34.5% (males) <strong>and</strong> 35.2% (females) was the parent<br />
spinosyn D (Domoradzki et al., 1996). Thus, for<br />
both spinosyns A <strong>and</strong> D a substantial amount <strong>of</strong><br />
metabolism had taken place in the first 24 h following<br />
administration. The primary pathways for the<br />
spinosyns in rats involved loss <strong>of</strong> a methyl group on<br />
the forosamine nitrogen (N-demethylation), <strong>and</strong>/or<br />
loss <strong>of</strong> one <strong>of</strong> the O-methyl groups on the rhamnose<br />
(O-demethylation) (Figure 4), both followed by<br />
conjugation. Conjugation <strong>of</strong> the parent was also<br />
noted as an important pathway in rats (Domoradzki<br />
et al., 1996).<br />
In lactating goats, following 3 days <strong>of</strong> dosing with<br />
either spinosyn A or D, eight metabolites <strong>of</strong> spinosyn<br />
A <strong>and</strong> five metabolites <strong>of</strong> spinosyn D were<br />
detected. As with rats, N-demethylation was an<br />
important metabolic route. In addition, hydroxylation<br />
<strong>of</strong> the macrolide ring was noted as a primary<br />
metabolic pathway (Rainey et al., 1996) (Figure 4).<br />
In studies with poultry, N-demethylation <strong>of</strong><br />
the forosamine nitrogen <strong>and</strong> O-demethylation <strong>of</strong> the<br />
rhamnose moiety were also found to be the two primary<br />
metabolic pathways (Magnussen et al., 1996).<br />
In the latter case, the 2 0 /4 0 -O-demethyl metabolites<br />
(i.e., spinosyns H <strong>and</strong> K, respectively, for spinosyn<br />
A metabolism) predominated over the 3 0 -O-demethyl<br />
metabolites. Another pathway, involving removal<br />
<strong>of</strong> the forosamine sugar to form the C17 PSA, was<br />
identified, but appears to be secondary to the N-<strong>and</strong><br />
O-demethylation (Magnussen et al., 1996) (Figure 4).<br />
As demonstrated by these three studies, spinosyns<br />
A <strong>and</strong> D are readily metabolized in vertebrate<br />
<strong>and</strong> avian systems. Furthermore, N-demethylation<br />
<strong>of</strong> the forosamine nitrogen is a metabolic route that<br />
predominates in all three species. O-Demethylation<br />
<strong>of</strong> the rhamnose sugar is also important in rats <strong>and</strong><br />
poultry. These particular pathways are both consistent<br />
with oxidative metabolism via monooxygenases<br />
<strong>and</strong>/or the action <strong>of</strong> glutathione transferases. In the<br />
rat studies, glutathione conjugates were noted for<br />
O-demethylated metabolites as well as for the parent<br />
spinosyns (A <strong>and</strong> D). The presence <strong>of</strong> hydroxylated<br />
macrolide metabolites in the goat provides<br />
direct support for involvement <strong>of</strong> monooxygenases<br />
in spinosyn metabolism.<br />
6.3.1.2. Spinosyn metabolism in insects Spinosyns<br />
A <strong>and</strong> D appear to have a number <strong>of</strong> potential sites<br />
for metabolism, including N-demethylation <strong>of</strong> the<br />
forosamine, O-demethylation <strong>of</strong> the rhamnose, epoxidation<br />
<strong>of</strong> the 5,6 or 13,14 double bonds, opening<br />
<strong>of</strong> the macrocyclic lactone, etc. As shown above (see<br />
Section 6.3.1.1), some <strong>of</strong> these potential metabolites<br />
have been observed. However, compared to more<br />
conventional insect control agents, the spinosyns are