The RPL21P90 gene is a pseudogene that is part of the ribosomal protein gene family. It is related to the protein-coding RPL21 gene, which encodes a component of the 60S ribosomal subunit involved in protein synthesis. However, as a pseudogene, RPL21P90 is non-coding and does not produce a functional protein product.

### 1. Associated Genes
RPL21P90 is associated with other ribosomal protein genes, primarily RPL21, which is the functional counterpart. In addition to RPL21, ribosomal protein genes such as RPL22, RPL23, and others may be indirectly associated due to their involvement in similar biological functions and ribosome assembly.

### 2. Aging and Age-Related Diseases
There is limited specific information connecting RPL21P90 to aging or age-related diseases directly because it is classified as a pseudogene. Pseudogenes often serve regulatory roles in gene expression or can be involved in the process of aging indirectly through their role in modulating the expression of functional counterparts. Ribosomal proteins and their associated pathways have been implicated in cellular aging and the regulation of senescence, but specific studies on RPL21P90 are scarce.

### 3. Drug Interactions
There are currently no known specific drug interactions directly linked to RPL21P90 due to its status as a pseudogene. However, drugs that affect ribosomal functions or translation might indirectly impact pathways involving RPL21, including antibiotics that target the ribosome such as macrolides, tetracyclines, and aminoglycosides. 

### 4. Pathways and Biological Processes
As a pseudogene, RPL21P90 does not directly participate in biological pathways like active genes do. However, its functional counterpart, RPL21, is known to be involved in:

- **Ribosome Biogenesis:** The assembly of ribosomal subunits.
- **Protein Synthesis:** Participating in the translation process.
- **Cell Growth and Proliferation:** Ribosomal proteins play a key role in these cellular processes.
- **Cellular Response to Stress:** Involved in the regulation of cellular stress responses.

### Conclusion
The RPL21P90 gene is primarily of interest for understanding the evolutionary relationships of ribosomal proteins and functional gene regulation. Given its pseudogene status, the known roles are mainly inferred from related genes, with limited direct evidence addressing aging, drug interactions, or specific pathways. Further research would be needed to elucidate any specific roles it might play in cellular processes or diseases.